Here's a way for you to explain it to them. As mentioned, the question at heart is about rate and proportion. With that perspective in mind, you can see how I set it up to be explained.
(I'm assuming that grass doesn't grow in the time spent, which would be too complicated otherwise.)
Ask them these questions. You can release them 1 at a time, or all at a go, depending on your aims.
- How long does it take 9 elephants and 30 horses to eat up a field? (Ans: 1 hour)
How long does it take 10 elephants and 25 horses to eat up a field? (Ans: 1 hour)
The idea of the first 2 questions was to get an equal time comparison (in this case I chose 1 hour). This tells us the relative rates of the elephants and horses.
Hence, 1 elephant is 'equal' to how many horses? (5 horses)
In simultaneous equations, this is the step of expressing one variable in terms of the other to eliminate it.
Hence, how many horses take it take to eat up a field in 1 hours? (75 horses)
Now, we actually find the rate of a horse.
Hence, how long does it take 6 horses to eat up a field? ($\frac{75}{6}$, I hope you taught them fractions)
Hence, how long does it take 1 elephant and 1 horse to eat up a field? ($\frac{75}{6}$)
Depending on the ability of the kids, I would leave out this question, or perhaps replace it with "Why do I care about 6 horses?"
Note that algebra is just a fancy way for shorthand. Back in the day, they worked with equations as
" The quantity when cubed, and added to itself thrice, is equal to four."
Of course, now we just say $x^3 + 3x = 4$. If they could do math then, it's not impossible to leave out algebra when talking with young children.