The kids are 4, 5, and 6! Reading is just a little above most kids of that age. They do however have amazing ability to remember things, especially with pictures involved.
Younger children tend to want more pokemon and less trainers (too many words), potion and switch are easy for them to understand and use so they are typically good to put into a deck for a small child to help get their feet wet in the game. The best draw card for a young child right now is Island Hermit, no shuffling, no deciding what to discard, and as a bonus they get to see their prizes too which will help them during the game. TVR is probably the 2nd best as there is no shuffling again. Prof. Cosmo's is another easy to understand draw card. Team Galactics Wager is a fun card if your kids like RPS, but it does involve shuffling.
Pokemon with straightforward attacks are best, so Lucario/Rampardos could be a simple beginners deck that shouldn't be to hard to grasp. The Mantine deck above is pretty decent, but little kids tend to forget they can't attack next turn. You could add Azurill, Marill, and Azumarill to the Mantine setup to have a more pokemon centered deck for a young player.
Once they get the general gist of what's going on Kricketune could be a good deck to learn on in a more competive way in tournaments as the concept of the deck is very straightforward. There are quite a few search cards and a lot of shuffling due to mulligans, so I wouldn't really suggest this for very young players.
