yeah, those games geared towards kids are cute and all when you look at the box, but the reality is that alot of those are really poor games. alot of people will say that younger children do not require games as complex or intricate as older children or adults, but that really is based on a child-to-child basis. i mean, pac-man was a huge hit in the 80's and all you did was move a little guy around the board to collect the dots. the genius in the design there was the artificial intelegence of the ghosts who case you, and that element made the game what it is, not just the maze.
thinking of that, if these are intelegent, interesting little girls who enjoy challenges as well as playing with barbies, then "The Sims" would be a great game idea for them, since the genius of design in that game is that you can get as complex or simple as you like and still have endless hours of entertainment, mindless or not.
i assume the shooting games are out of the question here, so the car racing games are always another option. stereotypically a boy's game, some of the racing games have alot of replayability and are really fun for everyone (women like to drive too

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lastly, puzzel games like the myst series can be tons of fun for kids, but often require playing with an adult at first to get them through the akwardness of the initial unfamiliarity with the game interface, and them some children just are not the types who enjoy that sort of thing anyhow. rule of thumb here, if they are good at math in school and enjoy the subject, they will probably like those games.
for the 4 year old i would have to stear you towards the educational games, which unlike the strict entertainment games seem to actually have a great deal of value in the children's software arena. the educational games i have seen seem to have had much more thought put into them with playability, graphics and ultimately have a much better value to the child. like the "where is carman san diego" games of over 10 years ago, which were fun to play even on their archaic interface, the newer educational games follow that same idea.
anyway, hope this helps. perosnally, i find alot of the kids games insulting to the intelegence of the children they are marketed for. the graphics tend to be sub-par, on 1998 technology, and there is no real depth to the games except for the educational ones. it's pretty sad that the majority of the game content out there now has to be geared towards adult subject matter, because an intelegent person realizes that with a computer game, it really doesn't have to be that way. but that's how marketing and sales works.
good luck!