I've done it for the same reason, I've got a small appetite too so I only order adult meals when my husband or a friend with a large appetite are there. Generally a normal adult meal isn't big enough for them, so when they eat my left overs they have enough to eat too

And actually, I find that when you explain at table service places, they tend to be very understanding and let you order a children's portion anyway.
The time I ate at the Sequoia Lodge they didn't make a fuss about it at all so it's worth asking and explaining you simply don't have a large appetite.
after all, the worst that can happen is them telling you no, but they may just as well allow it
