When Gabe runs into Rosemary as she and the nanny are taking her little sister to a birthday party in Central Park, the sister is in a stroller. Afterwards, when they all meet up again, there is no stroller.
As has already been mentioned, David Betenahu was in the preschool photo with both Gabe and Rosemary, as well as being visible in the Karate class on the first day. But he is also shown playing the trombone beside Gabe when Gabe tells us about his life being filled with song, sport, and constant adventure.
Gabe claims that he didn't know anyone else but Rosemary on his first day of Karate class. But later when he gets partnered with David Betanahu he comments on how David has had a mustache since nursery school. And the photo shown of young David is the exact same class photo that included young Gabe and young Rosemary. Since you can spot David among the others in the first day of karate class, it can't be argued that David joined later.