MarcyinPA - TRU does not always have a great selection of games. It might be helpful to you to check some websites - such as
www.ebgames.com www.gamespot.com or
www.ign.com - to find reviews and ratings so you'll know what games you want to look for.
Gamecube is VERY family friendly. I always hear it called "kid-friendly", which it is, but I am 31 and I love ours. Most of the games are "cuter" looking than those for the other consoles, and they are less violent for the most part. The Mario games, particularly the sports titles, are great. So is Animal Crossing. My son has his own Gamecube (because he always wanted to play while my husband or I was playing!

) and his Christmas list is full of Gamecube games.
X-Box is not very child-friendly. We only have one game for ours that my 9 year old is allowed to play. Most of the family-friendly titles for X-Box are also available for Gamecube or PS2, so we just get those instead. The graphics are great, but that doesn't help if there's not that much to play.
Most reviews I have read for games that are on all 3 consoles rank graphics and gameplay as: 1)XBox 2)Gamecube 3)PS2. More games take full advantage of the PS2's abilities than the Gamecube's, though.
My family has all three, and we by far spend the most time on the Gamecube. We also use the PS2 quite a bit - especially online against friends in other towns. And of course for Kingdom Hearts, which is why we bought the console! Katamari Damacy is also quite addictive.
We got the XBox for Knights of the Old Republic (and now KOTOR 2) and that is almost the only thing that it gets used for!