I know of what you speak. Children always want what the another has (I know of some "adults" that are very much like the children). You're very wise in taking them outside and wearing them out, I often do the same when our grandsons are here (sometimes tis I that gets worn out first).
Hugs!
You have to be careful when trying to wear them out. There is a fine line between wearing them out and getting them in shape!! More stamina in a 20 month old and 3 1/2 year old is not a good idea, and not for amateurs. LOL!