We did that for Corin, Jr. when he was 9. We thought especially since he was an only child he needed a dog. And we got a big dog (Black Lab). Nobody told us how much those things shed.
Had we known that Corin III was coming along, we might have opted for a smaller dog. But "Sam" (sorry, named him before we knew ya - full name is Samuel Adams) is very gentle with the little one, and very tolerant. Still we know he's an animal, so we don't leave them alone.
But I no longer have to tell the dog to stop barking when we leave him out or ask him if he needs to go outside. The 2 1/2 year old does that for us.
We're a little concerned because we've found out the older one has allergies and the little one has asthma. So far though, we're doing okay...