Being a CA girl and growing up on the beach one of my all time favorite things is a bologna and mustard sandwich...even with the sand...adds a lil' crunch!
All the ideas you have so far will work great...no matter how hard you try your kids are going to eat sand with their food, its just the way it goes! I 2nd the grapes tho, they are an excellent beach food! Go-gurt would work pretty well too!
As long as you are taking the cooler, you can pack anything you'd like!
~We just got back from visiting my family in South Gate (near Long Beach) and stopped in at Seal beach to let the kids play...my oldest nephew got stung by a stingray...ouch! Apparently that is VERY painful!! I am 33 yrs old and spent most of that on that same exact beach and never once got stung...only my nephew, jeez, can't take that kid anywhere! So, moral of the story...it doesn't hurt to stop by the lifeguard station and ask about your current location!
Have fun and enjoy your trip!