Wifi will probably be available on the plane, but you'll have to pay and wifi performance will be substandard.
Probably no seatback screens. Even if you're on a 737-8 you probably won't have seatback screens. If equipment changes to an EMB175, you still won't have them.
Movies can be watched through paid wifi, but you'll need to have the United app installed either on your phone or your laptop. It would be courteous to all concerned to watch movies with headphones on. If, as your post implies, you will have more than one kid with you, only one kid at a time would then be able to watch.
You may get a free non-alcoholic beverage, possibly a snack mix. No cookies. No food cart. Probably just water, sodas and coffee. Eat and drink before you board.
So much misinformation. Obviously, if the parent has three iPads and/or phones, all of them would be able to watch movies (parent + 2 kids). Just need to download the App on all devices.
Food can be bought. Flights scheduled over 1.5 hours have a snack shop all day. Flights scheduled over 3 hours have Bistro on Board (with Breakfast items between 5:00 AM to 9:44 AM, and lunch & dinner items between 9:45 AM to 8:00 PM). To make sure to have all the choices available it's better to be sitting closer to the front. DFW to SFO should be in the range for the Bistro on Board.