Half Moon Bay's Main Street is a lot of family friendly fun. The stores aren’t really for children, more for parents, but kids will still enjoy the old timey feel of the street.
There's an awesome hotel located directly on Main Street that my family stayed at called Half Moon Bay Inn. I can't recommend it enough. Our suite had its own private entrance and was very large. We even had a full kitchen and dining room! Each morning when we left our hotel to explore Half Moon Bay, it was so much fun to step directly out on to Main Street.
Main Street is small but eclectic. You can tour the whole street in a few hours. With that being said, do it! There’s a larger than life, beautiful mural on the side of a building for kids to admire, a yummy restaurant called Its Italia to eat at, and lots of free parking. When in Half Moon Bay, Main Street is not to be missed.
The previous reviewer is right about main street shopping not being altogether child friendly. However, there are a few exceptions. On the east side of the street is Half Moon Bay Feed and Fuel, a throwback to the cowboy era. The front of the shop is crammed with pet and livestock supplies (and a bit of touristy kitsch). Bring your kids to the back of the store to see the baby chicks, bunnies, ducks and occasional guinea pig.
I have purchased baby chicks here, as well as chicken feed and various poultry supplies. There prices are very good and my daughter loves to see the babies!
As you would expect in an upscale main street, there a lot of boutique shops that are not that kid friendly. I would recommend taking the kids to the local beach and then to one of the local restaurants and/or window shopping, but you won't be able to do any serious shopping with kids in tow. Once a year they have the pumpkin festival which is fun and a lot more kid friendly.