Ours were 5 and 7 and had no problem whatsoever. The heat in August isn’t as overbearing as people here are suggesting. Plenty of shade and air con in NYC and you plan an itinerary based on your needs!
Up early in the morning and out to sightsee, I don’t rate the New York passes, especially with kids. Much better to think about what they need, what you like and work it that way.
The library, grand central station, Staten Island ferry, picnic in Central Park, hook and ladder 8, the high line, Times Square (first thing in the morning, skip at night, it’s a shithole), Bryant Park (free games to play and bands/festivals), Chelsea Market, walking Brooklyn bridge. All of the above are free, and with that to do why would you spend on expensive passes that restrict you to a time slot?
It is busy, never empties out as others might have you believe, and is hot in summer but more than manageable. Kids will love it as long as you don’t project your anxiety about getting lost on to them and yes, train them to stay with you as they should anyway.