Hi! Long-term Marylebone resident here. Have lived here since my youthful single days all the way through to now having a family. My DD is at QCPS.
The area is family-friendly for sure. It is well located for both Regent's and Hyde Parks for outdoor space, and you can also sometimes rent a property which could come with access to one of the private garden squares if you want outside space. Lots of weekend kids sports clubs run there. It is so easy to get around so activities such as all the museums, galleries, theatres are all also in easy reach and take no time at all to get to. And easy to get to the surrounding more "suburban" neighbourhoods like St Johns Wood. The local leisure centre has climbing and a pool along with gym, sports hall etc - though it is being redeveloped soon, but hopefully means it'll be new and shiny in the next few years.
Community-wise there is a large Marylebone Mums and Dads community set up years ago by a local mum that is on FB as well as an active chat group on Telegram where there are local families supporting, chatting, sharing info. People on there are really welcoming as many are expats or here without family around so its a really friendly supportive community where you build can friendships.
There are a few streets other than Marylebone High St that have shops and cafes like Seymour Place and Crawford St, and lots of independent businesses all about the area. Around the High St and the part West of Baker St is very residential - you see the same people every day, and it very much feels like a community. There is also the local Marylebone Association that works to ensure that the community vibe persists, as the location of the area means there are always attempts to redevelop everything!
Long answer to a short question! But its a great community.
You obviously like QCPS - its a fab school and you'd be lucky to have it as your main local option for private education.