We don't sail with kids, not that we mind them.
I don't believe that kids would feel unwelcomed on Celebrity. Or on HAL, for that matter. If fact, we sailed on HAL with a family with two kids, and the kids loved it because of all the personalized attention they received.
You're right -- Europe is so port intensive. Our last
DCL cruise was the May 2013 EBTA. Upon disembarking in Barcelona, we stayed 3 nights, then sailed on a NCL cruise from Barcelona to Venice. Through our DIS cruise meet thread, we met two families, with kids, who were doing the same thing.
The NCL cruise was port intensive: 12-nights with just 2 sea days. My DW and my typical routine was to wake up early, have a quick breakfast, enjoy a full day of sightseeing, get back onboard, have dinner, maybe see the theatre show (I skipped it most nights), go back to the cabin and read up on the next day's port. There wasn't much time for anything else.
In talking with the two families, sailing on NCL worked very well for them -- especially the Freestyle dining (no set traditional dining time). Guests could come back onboard and eat when they wanted.
HTH
Woody