We went to NYC with Katz last year. The girls were 7y, 6y and almost 4y at the time.
There is loads to do with children in NYC and we didn't get it all dne in the week we were there.
What we did do:
Children's Museum of Manhattan - good half day activity
Natural History Museum - spent nearly a full day here and didn't see it all
Central park - you could be in here for days; there are lots of playgrounds, as well as lots of green space to run round and rocks to climb' great for a picnic too
We got a 2 day hop on/off bus pass which was a nice way to travel round, see the sights and get from one place to he other.
We went up Top of the Rock. We went to Ellis and Liberty Island. We also went in the Guggenhiem - but to be honest I'd skip that and try the Met next time.
We are taking DD (now 8y) back in July as part of our summer holiday.
We found NY very very friendly. We stayed out of NYC and travelled in every day ont he train. We never had to stand. The minute we got on with the children someone would always get up and offer us a seat if none were free. They don't take no for an answer, lol. If you have a map out someone will come and ask if you are ok and need any help. One woman even walked way out of her way to show us where to catch the train from. It was the friendliest city I have been too TBH.
Have a fab time.