You can get Q/R axlesin all sorts of thread dimensions, so I'd recommend taking your kids' wheels into your LBS and seeing if you can find axles for it. Easiest way is actually to just take the locknuts in and find Q/R axles that match them. Local Bike Shops with good mechanics are great for wide selection of small parts when you need to match something that you have in your hand.

One possible worry: lots of kid's bikes have very thin dropouts, which will require you to get a very carefully-measured axle length. If the Q/R axle sticks too far beyond the outer edge of the dropout, you won't be able to shut the quick-release skewer.