Here's a link for pet friendly accommodations:
Myrtle Beach Pet Friendly Hotels Lodging Condos
There is a parking garage in downtown Myrtle and there are a number of places you can park on the street or in little side areas that lead to the beach. I think all are metered or require payment as you leave.
When my kids were 3 and 5 they really enjoyed the animal things: Alligator Adventure at Barefoot Landing, Ripley's Aquarium at Broadway at the Beach, and the Waccatee Zoo on Enterprise Road off of Route 707 below Socastee. Here are some links:
Welcome To Alligator Adventure || Reptile Capital of the World!
Ripley Aquarium of Myrtle Beach (http://myrtlebeach.ripleyaquariums.com/ - broken link)
Waccatee Zoological Farm - The 'Beast' Kept Secret In Myrtle Beach
The Children's Museum of South Carolina is an excellent hands-on place for kids to touch and explore and do things:
Children’s Museum of South Carolina, Myrtle Beach, SC
Of course, there are TONS of miniature golf places to choose from. Dragon's Lair at Broadway at the Beach (BATB) is pretty cool for kids. There are many that are pirate themed and many that are animal themed. There's a nice little playground area at BATB also, which is nice to take a rest from shopping to allow the kids to get rid of some excess energy.
Brookgreen Gardens is also a nice place to visit. They have a little zoo and a boat ride, both of which cost a bit extra. Your ticket to the Gardens is good for seven consecutive days, so if you don't get to see it all, or want to go back another time, you can.
Brookgreen Gardens Myrtle Beach -- Ever Changing. Simply Amazing.
There are also two oceanfront state parks: Myrtle Beach State Park and Huntington Beach State Park (right across from Brookgreen Gardens).
There is so much to do that you can't possibly do it all in one trip. Of course, a lot depends on the weather when you're there, but if it's raining in one area (say the north end) you can often head to the south end for the day as it probably won't be raining there.
Enjoy!