I agree that their names (probably) won't change, and your own children are exempt from any "no name tattoo" rules. If you like tattoos of words, just get their names. (And maybe leave room so your third child doesn't feel left out ;) )
This is my tattoo "for my son". If I got it based on his interests, I'd have Blues Clues, a tank, a monster truck, or any one of a million video game images. It doesn't have his name either. I know his name. :)
This is a wood sorrel, and in the language of flowers it means "joy and maternal tenderness."
But beyond "what it means", I have a tattoo I really like.