Do you think they'd enjoy the MoMA?
If you're asking if the OP with her 8- and 5-year-old daughters would enjoy MoMA, then I'd have to say it depends on the daughters. Only the OP and her daughters would know if that's something they'd like. I'm guessing, though, that it might take an unusual 5-year-old to get into a de Kooning show, which is the featured exhibit at MoMA right now.
I'd recommend the Metropolitan Museum, though, since the Egyptian collection is just spectacular and it's something that anyone of practically any age would enjoy. Also, the museum itself is a great space. A very well-kept secret of the Met is that it's open on Friday and Saturday nights until 8:45, and the galleries are far less populated than they are at any other time except at rope drop.
But if the OP and her DDs are in NYC just for the day, going to AGP and either FAOS or TRU, skating at Rock, walking around, and having a big meal will more than fill up their day. The Christmas show at Radio City for the evening would be excellent. Just seeing the interior of Radio City Music Hall is awesome, even without seeing a show.
One of the most fun things to do in NYC--and it's free!--is to people-watch. There's no better place on earth for this. If you can avoid overscheduling yourselves, just taking in the scene here is a terrific experience.