If you go for a wetsuit style top you'll be very toasty. Have a look on Surfstitch they often have specials and you can try a few different styles at a much reduced price.
I like to use a standard rash vest but i wear it tight, i also like to have a draw string at the bottom to stop it from lifting up. I tend to be grovelling around in the sand a far bit though. I guess if your out with your kids you'll be similar.
You can get Fluffy or extra warm rash vest also. We picked up some from Westsuits years back and my wife swears by it. Too hot for me. From memory though it didn't do too much for wind resistance.
Id go the wetsuit material option..