I have one bike and it’d be highly inappropriate for one of my kids to have more…
Sounds like a fine opportunity to n+1 up to me 🙂
My son (8) does a bit of everything and at the moment he has an MTB, a road bike and a CX / grass bike. In truth, the road and CX bikes could be merge but they were dirt cheap, so I don’t mind them being separate at the moment. We have been an MTB family for years, so I would never be without one, but we now both do a lot of road riding and really enjoy and I don’t think you can really do both justice with a single bike. One thing’s for sure – if he’s riding his road bike, I certainly couldn’t keep up with him on my MTB any more, which more or less mandated me to have one too!
Next year, I’ll build him a less rubbish CX bike and perhaps a new road bike, but for now what we’ve got works ace and didn’t cost (too) much money. Kids love riding, so give them the tools to do it if you possibly can 🙂
Personally, a half decent MTB and a Luath (26 or 700c depending on leg length) would seem a really flexible combo and cover you for pretty much any riding.