Santa Maria Valley Discovery Museum - Younger children would have a great time. Older children not so much. The only redeeming thing for an older would be an area where one can make whatever with various recycled items and hot glue (provided). However, I know they have overhauled some areas since we were there last so I don't know how it has changed.
The Santa Maria library is very close by and is great! A big children's area. And the mall is nearby also. It's not the greatest, but there is a pet store that is fun and a great cookie place upstairs and a Whipper Snippers - good for inexpensive haircuts - ask for Mariana - she is the owner and she is the best. The kids can pick out a movie to watch while they get their hair cut and some of the little kid chairs are cars and planes. They even have an adult chair and a basic cut without a wash is $18. There is also a Red Robin at the mall, and they are working on a brand new movie theater.
Santa Maria also has Target, Toys r Us, Olive Garden, Red Lobster, Wal-mart, TJ Maxx, Foods Co, JCPenneys, and Joanne's Fabrics, which are all lacking in SB, I believe, so that may be of interest to some people. Them there are places that are also in SB, but perhaps good to know about anyway: Costco, Best Buy, Panera bread, See's, Trader Joe's, Lassens, Michael's (it's smaller though), among others.
Waller Park - lots of green grassy areas, a very large two-part pond with an island of geese and ducks, swings and a large play structure. They also have pony rides further back into the park on the weekends - has a permanent location with an old west theme. I think the rides are $3. Warning - probably a lot of parties at this park during the weekend.