hi - we have been there a few times on NYE. I think it really depends on your family, and what you are looking for. We got to the MK early, and just camped out there all day long. It is VERY crowded - very tight pack for sure. We had alot of fun, but we knew what we were getting in to. We had a few people " camp out" to hold a spot on mainstreet for the parade and fireworks, and 1 year i think they even cancelled the parade because it was too crowded,,,i cant positively remember. They passed out hats and horns and people had lots of glow and light up necklaces and stuff - it was a party atmosphere for sure! we had a ball -
the lines were very long for sure - but after midnight, the park was still open for a few hours, so we went on rides then,,,what a way to start the new year with doing the mountain range at the MK!!!
now, if you think your 7yo isnt up to an all day affair, you may want to skip the parks and do some other things - maybe a water park or disney quest or rent a pontoon boat or just enjoy your hotel or something,,,if they fell asleep, carrying them through the park in that crowd would be awful - we also had to wait a long time for the bus to get back to the hotel...
you will have fun whatever you decide to do!!!