Your fears for your children potentially becoming fully indoctrinated are justified. They're still young enough that if you provide true balance in their lives (extracurricular activities outside the JWs) that they could be reasoned with.
One of my favorite comparisons that my eldest understood right off the bat when we decided to leave the JWs, was talking about Santa Claus as if he were "pretend". A lot of people tell their kids that Santa Claus gives children whatever they wish for at Christmas, if they are good, but if they are bad he gives them a lump of coal. Children write letters to Santa asking for their special gifts. The parents peek at the letters and pretend to mail them to Santa, and then become "Santa's Helpers" and buy the gifts the children want. Santa is pretend, but parents want their children to be happy and believe in a benevolent stranger who loves them, when really it's the parents who love them so much.
Then we talked about Jehovah. He tells us to pray to him (make wishes) for the things we want. If we are obedient (good) apparently Jehovah will answer our prayers. If we disobey, he won't let us live in Paradise™/will destroy us at Armageddon™. He knows if you've been bad or good, and he's making a list and checking it twice. He even sends a man with an inkhorn to mark you in the forehead if you're good. He doesn't have a team of flying reindeer, but he does have a massive chariot in the sky with wheels that have eyes in them.
Do reindeers fly?
Do chariots fly in the sky? Do they have wheels with eyes in them?
Are they real or pretend?
That being said, it's still important for children to learn values of morality, justice, compassion, respect for others etc. You can foster those by volunteering with them - at a soup kitchen or food bank or helping a local family experiencing hard times. When they grow tired of toys or outgrow their clothes, teach them about sharing by giving those toys and clothes to a shelter or disaster relief center. While the WTS wants their focus to be on the WTS, help them expand their world view and your own by helping the less fortunate.