I've always considered Valve a solid developer. They've created classics and shown they can innovate. It's unlikely they are going to make a game centered solely around the 7-10 year old demographic. What's more likely is they will make a game that has a high level of accessibility so that the 7-10 year old demographic can play it.
What is also unlikely is that they put very little effort into any title they're making. Simply because they have important franchises to maintain does not mean they say, "Hmm... let's pump out a game for the Wii, we can make a quick buck. Go find Larry and Joe and a group of day laborers and set them to work! Pronto! We have no time to waste." Granted I'll agree that this is something you could hear from some shovelware developers, but Valve actually has a reputation they'd like to uphold.
