I have 2 boys 6 & 7 and they have both. The GBA SP is in the drawer collecting dust and they would play with their Ds lite daily if I would let them. Their favorite games are Mario hoops 3/3 , mario cart , Nintendo dogs and some pokemon game. I however use it to play brain age ( I have become addicted to suduko ) Plus the can link up with other people and versus each other and they can go into picochat and communicate wirelessly.
DD6, received hers from Santa one month before turning 5. She LOVES hers and gets tons of use out of it. She had the Gameboy Advance SP, but She LOVES her DS. (AND, as many PP's have said, you can play gameboy games in the DS, not vice versa). I would get the DS for both of your children and I am sure you will not go wrong.
We were just talking about this yesterday. Both of my sons (ages 6 and 8) want the DS versions. I was told that only the cool, new games are available on DS. It's always the newer version that they want, though. They both have the Game Boy Advance versions and still want DS because of the new games.
The DS for sure . We have both and the DS has a better picture and looks more realistic. My DS wanted Dogs which is really cool and rather realistic looking. He can feed, pet, train, do agility games and all kinds of stuff with it. My DD got the old one and we bought her the Advance version Dogz and compared to the other one it sucks . It is very cartoony and she can't talk to it( you can on the DS)or really do much with it. She has been very disappointed and I think we will have to break down and get her a DS (my DS got his from his grandparents for Xmas). Every time she sees her brother's she cries about the fact she wants the DS too. With all the advances in the technology you will be better off spending the money and I can guarantee you that all the really good new games are going to come out for the DS and not the Advance or as in the Dogz it won't be nearly as good.