I'd say that the thing to emphasize is that it is about comparing. It's relative. Both numbers must be accounted for in one view.
It's not about how big either number is, it's about how they relate.
You can't look at just the top number or the bottom number alone. Are 3 pennies worth more than 2 nickels? You can't say 3$>$2 and be done. And you also can't look at pennies$<$nickels and be satisfied (since of course 6 pennies $>$ 1 nickel). You have to look at both parts together.
Emphasize this phrase/concept... it's about how full it is. Show her a big auditorium or stadium... if 20 of the seats have people in them, would you say it is very full? More so than 12 people packed into a tiny room or a closet?). It can be something you keep coming back to in your everyday life until she's got it. Do you feel like this bus is pretty full? Is this parking lot more completely full than that one? Is this bowl of soup more full than that one? Is your backpack more full than that cubby hole? Is this notebook quite full? That helps appreciate the idea of comparison (and build up better insight into the relative sizes of fractions/percents... one of the most useful foundations to interacting with our everyday world throughout our lives)
It quickly comes to the fact that while adding more items on top (increasing the numerator) DOES make any situation to be more full... adding more slots to the bottom (increasing the denominator) makes it LESS full. So if you want to go that direction, you can help her understand making the bottom number bigger actually winds up always making a fraction worse. But emphasizing what's key overall is not really how many pieces there are, how many "slots" there are, or how big each piece is, but about the completed picture, how "full" it really is in the whole combination, to really aid understanding.
The best help for quite a few kids may indeed be the classic shapes. The pie diagram (or a similar block diagram). The better, winning, one is the more "complete", full drawing. Is 3/5 really more complete than 2/3?
If you'd like larger images, which could be printed and then laid on top of each other, here are 3/5 and 2/3.
If you want to hammer home the point that it's not about numerator size ask her if like 25/100 would be more full and look at that picture. It has got a lot of slices (or seats in the room) filled in. But it's got an awful lot of empty ones too.
Now, if you're asking specifically about comparing 2/3 with 3/5 as the specific skill, indeed because they are so very close in relative size... I'd suggest that there may not be any very useful way for the an 8 year old to directly compare them without being given the picture beforehand... and that's not entirely a bad thing. It's good to show people there's questions they can't answer, to make them look forward to expanding their horizons, and learning new tools. Recognition of those fraction sizes comes with practice of seeing them, just like tying one's shoes.
That's why percent/decimals are to come very soon on behind this lesson in the curriculum. It may well be coming up right after this workbook. At most, I'd expect it's about a year away.
Until then? I'd suggest the only hopes are memorizing or visualizing it. But that it's really not a question she needs to be pressed hard to be perfect at yet. If she doesn't remember, she could try making careful drawings of equal sized slices/blocks, and having her see if she can be precise enough to estimate which one is more full. But indeed these particular fractions are just so similar it's pretty difficult to draw - I for one certainly found thirds in particular to be very hard to draw well as a kid.
You could try looking at both pictures in 15ths... but I think that the math to make sense of that can lose many children, especially if not explained with great care and precision. And it still wouldn't be a very useful tool for answering such future questions for a typical 8 year old. It would introduce conversions/common denominators and a lot of useful math... but I'm a firm believer that you REALLY don't want to get kids too lost trying to perform complex arithmetic for a problem too early, before they understand the basis imagery fully enough, or they can end up getting lost in the blind "because I'm supposed to" instead of really having an understanding of what they're doing. She really has to understand that fractions are relative comparisons, all about fullness, before she can best wield the arithmetic on them sharply and to large benefit.
Indeed, I'd argue that this question isn't given to students at this age seeking so much for them to answer well. But that instead the point very much is: "this is pretty tough to get precise enough for, and that's why we need another way". Making children yearn for that way, and therein look forward to and welcome decimals/percentages (as well as fraction math) is a most beneficial thing. Indeed, these coming topics are probably the one math subject that the greatest percentage(!) of students struggle with in all of basic schooling. Especially before algebra. When I was a student, we spent three or four years going back and hammering at them... and many still didn't get them. There's a lot of different concepts and situations to learn to deal with in fraction/decimal/percentage math, and that can overwhelm many. And so having a great foundation on what the basic ideas mean really aids in learning when to use what. Plus, then being good at these will offer a solid foundation for better success in algebra and beyond.
So no need to cause undue stress. But instead an opportunity to reroute it towards encouraging a better understanding of how "full" things are, a tool which cannot be underestimated. Soothe her that it's OK if she doesn't easily/correctly get the answer to some of these more difficult questions for a little while. And her frustration now will prove useful in the long run. As long as you keep encouraging her that she has the tools to answer most such questions (such as by using rough sketches), she'll benefit both now and later to be seeing a question like this!