Locationwise for touristy things, Marble Arch is better than Maida Vale. But if MV offers a full brekkie & a cab ride each day (presuming it's not to your office), that might be worth it.

But I think the big thing is the 3yr old needing a nap each afternoon, in which it's probably easier to get back to MA. Plus, MA has family rooms that sleep 4.

If the train to Paris leaves from St. Pancreas, there's a new Ren there although no idea on the cost. So you could possibly switch to there. Otherwise, it's a 10-15 minute walk to GS, GH, PL or a quick cab ride.

Cheers