Couple observations...
Most of the "main myrtle" and south myrtle beach are a dump. It's old, it's run down, it's trashy. I didn't really know what to expect having never been there. But that area is about 40 years past it's prime. The beach itself was nice and the water temp was almost perfect. But the rest of it....meh. Think of the old part of Vegas and sad looking hotels and psuedo-high rise buildings.
It's got most of the touristy trap stuff...boat tours, a billion advertisements for the "Pirates" dinner and some for Medieval times. Shows, ripleys museum, ect. We were there for a week. I've got a 4 and 8 year old. We have a timeshare through Hilton and our property got us access to the water park south of town. It was fun. Several slides, a tide pool, lazy river, and you can upgrade to the go-cart pass. Might be worth it for a day. Crappy part was the go-carts didn't start until 1PM.
We also went to the Brookgreen gardens in Conway. It's something for either very young kids, or adults with no kids. Your teenagers will find it lame. Went once, won't go back again.
A big suggestion to me by someone was Broadway at the Beach. It's off Kings highway on the south part of Myrtle. We dove past it, saw about a billion cars and said NOOOOOPE. If you have older kids it might be worth checking out. Didn't want to take a 4 year old through there who hadn't had a nap that day.
Dinner wise, depending on where you are, keep heading south to Murrells Inlet. There's about 10 restaurant places staggered along there that you can walk through and look at. There's little island there called goat island that actually has goats on it

It's super busy so either get reservations or be prepared to wait. We stopped by one place at 6:00PM and it was a 2 hour wait for a party of 6. Booo.
If you get a chance out for "Mom and Dad" there was an amazing place we ate at on the northern end of Myrtle called Fire and Smoke.
http://fireandsmokegastropub.com/
One of the best meals I've *ever* had. We did a series of the small plates. The scallops were amazing.