Hi, I've used both the Medtronic pumps, not had a problem with either. The 640g (my current one) is the newer one, its waterproof and I personally like the interface. I'm not a parent.
The only issue I would think with the Medtronic pumps is the basic remote functions. The medtronic remote is very limited. The 640g remote (a Bayer Contour Next 2.4 Glucose Meter) can only bolus units and the remote doesn't give access to the bolus wizard or any other pump information. I imagine as a parent you would prefer to have access to information to what's on the pump. The glucose meter itself is quite good though, not too big a blood sample size and it sends the value to the pump automatically.
I've never used the Roche pumps but from what I've seen their remotes are a lot more full featured and give two way communication to the pump, so you could have a lot more control over the pump that way.
Anyway I've never had any problems with the Medtronic pumps and I love my 640g, you have a decent selection infusion sets and 2 reservoir sizes (I'm not sure what Roche pumps have but I imagine they're quite similar, ill let someone else chime in who's actually used one). The remote functionality could be a factor if you think it would be useful.