Hi Dawn, Sunny Beach is the bigger of the 2 resorts and is spread out over a much larger area. It is a very flat resort whereas GS is quite hilly in places and you need to check out where the hotel is. Both resorts have nice long beaches and the water is safe for children.
Both resorts have plenty for children and both have large cities near to them for easy shopping, parks, etc.
We have ourselves decided to back to GS this year after a number of years in SB with our hotel ( The Ozbor ) right on the beach just away from the main action.

I would be inclined to say GS is the best choice for a first visit and also worth a look is Albena, very friendly for children and less crowded than the main 2 resorts.

Toonman

some info on Golden Sands and Sunny Beach. But I'm sure members who have been to both can also add to this

May I add to Toonman2's remark about Albena, many of the Hotels are on the Hillside and you will need free buses or the noddy train for transport. look at Albena map and if you decide on that resort, just shout for help. Tonyt.