San Diego is the best place to go to the beach in CA b/c we have the warmest water on the west coast, usually around 70 during summer. Anything north of LA will have really cold water and will be too cold to swim in. Hotels during summer in SD are more expensive b/c that’s peak season. But if you stay inland it would probably be cheaper and if you want to be adventurous you could stay in a hotel in Rosarito and drive into San Diego for the day. Hotels in Rosarito are much cheaper b/c it’s Mexico of course and it’s not that bad of a drive between there and the border. Imperial Beach is next to the border and the cheapest beach community in SD so that would be your best bet for the cheapest hotel right on the beach.
SD has great little funky beach towns like Pacific Beach, Mission Beach, and Ocean Beach. Mission Beach would be great for you guys b/c it’s classic SoCal beachtown and kind of brings you back to what it was like here in the 50’s and 60’s. Also there is a Belmont Park which is great for families and cheap, there is a roller coaster there too.

I still think SD is pretty reasonable compared to LA, Orange County, and especially Monterey. YOu can get a room probably for $100 or less a night during the week. But getting a hotel right on or within walking distance to the beach will be more money, probably at least $150 and up a night. If you don’t mind staying a little farther from the beach check out hotels away from the beach like 5-15 miles inland in places like El Cajon, Mira Mesa, La Mesa, and areas around Mission Valley. Mission Valley around Hotel Circle has tons of hotels so you might be able to find a deal there.