Not to discount my learned division brothers but I would go the PS3.
(Give it a second for the XBOX boys to stop yelling traitor at their computer monitors
1) Although the most expensive of the three initially, the PS3 is the most complete and robust of the three consoles. It is the only one with Blu-Ray, DVD, Bluetooth wireless network access and flexibility regarding peripherals and upgrades. (You can use any USB keyboard, microphone or head set. On top of this you can upgrade the Hard Drive with almost any over the counter lap top Hard Drive and you don't need to know anything about putting computers together). Prices for the PS3 have come down quite a lot, but you are still looking in the $500 range for the console). There are hidden costs with the XBOX (No wireless networking without purchase of an adaptor, only propitiatory hard drives can be used, no Blu-Ray at all, etc.) and the Wii is only a games console.
2) Having appropriate material for you child to play is one thing, but you will want to use it as well. As PC Gamers, after the novelty of the Wii wares off you will find very little to keep you interested unless you are hard core Nintendo fans. There are many games available for the PS3 that will be appropriate for a 5 year old (EyePet
would be my first recommendation). So for over all family use the PS3 is miles ahead of the Wii and has some advantages over the XBOX 360.
3) Accessing the PSN (Playstation Network) is free. XBOX requires a monthly or yearly subscription to play with others online.
4) The Online Playstation store has more games for kids that than the XBOX Market place IMHO and they give costs in real money, not Microsoft Points so you have control and a real understanding of what you are spending.
5) The PS3 is launching a Wii like motion controller this year, with similar games to that on offer by Nintendo. So you will not miss out on Wii style games by going to the PS3. As PC gamers you will not be impressed by the Nintendo's graphics I fear and as a family of gamers, I think you will want more choice than the Wii can offer.
The PS3 is the Hub of my families home entertainment. It is used to play our movies, our music and most of our gaming now (we use to be very XBOX focused ... but that is changing). It is even our PVR as with PlayTV
we use it to record TV shows.
If I was forced to pick one console for my family, it would be the PS3.
My two cents for what it's worth