Okay I second all the suggestions. First we all know that Children have to "Catch" or they will be bored. Size does not matter at all, it's the feel of a fish on your line. Here are some comments from a local resident and professional charterboat captain. The Little Bridge or Jennettes Pier would be the easiest for small children, No Casting Involved. Unfortunately the Little Bridge gets significantly slower during the day. Early Morning or Late Afternoon is Best!. Jennettes Pier, for children, Bottom Fishing is best.
Unless they are able to cast underhanded I would forget about the Blues or Spanish. From my experience the best Bottom fishing is about 2/3rds out just past the last Sun/Rain Shelter. The North side of the pier has parts of the old pier still in the bottom. The structure holds small bottom fish but expect to loose some rigs. There are some nice Triggers caught here along with Pigfish, very small Croakers and Spot. Directly across on the South side there is a small slough in the bottom but you have to flip out your rig about 50 yards. Depending on Tide stage it can be productive for Sea Mullet, very small Trout and an occasional Sand Perch. Bloodworms are the bait but soooooooo expensive. Shrimp will do the job quite well. Stay away from Squid which is inferior to worms and shrimp for this type of fishing. The Little Bridge has Rest Rooms and naturally is Free. Jennettes is the most expensive of the piers, something like $12 for adults and a bit less for young children. Jennettes has nice Rest Rooms, a nice Beach Adjacent to it with Bathhouse (for the non fishermen in the group) and the Pier has Security and Fishing Staff that can help beginners. The Pier under the Pirates Cove Bridge is good at times but you generally have to cast a bit. For the more Adventuresome you may want to try a Headboat (About $50 per Adult some have Children's Prices). There are Headboats at OIFC, Pirates Cove and OBX Marina. Good Luck