Children see through phoniness, arrogance, and egotism better than anyone else. Nole is the antithesis of these qualities, and Federer is the embodiment.
Children don't have money though and all companies care about is who will buy their products based on their ambassadors. Even if Novak was more popular with kids than Nadal or Federer (debatable) it would make no difference in his endorsement potential.
As for that photo, it's very cute -- though Nadal had a similar reception when he played DC in Belgium -- the crowd seemed happy to support Spain half the time. So it's hardly proof positive that Novak has some unique connection with kids that the others don't.