Took the family to see this yesterday. It was their first movie in the cinema. My daughter is 4 and my son is 2.
I'd say Lego Ninjago was 'ok', not great. I've seen The Lego Movie but not Lego Batman, I think the Lego Movie is better, the plot of Lego Ninjago is pretty rote. There isn't anything great about this aside from the animation. Jackie Chan as Master Wu was a bonus. Garmodon's voice actor is fantastic. The real disappointment is Lloyd who is perhaps the most boring hero I've seen yet in a kid's movie. The mechs are a real highlight and they're better than anything in the Transformers.
The kids enjoyed it, but Moana, Tangled, Frozen, Big Hero 6 are streets ahead.