Cambrils: pretty old port, little pedestrian lanes with lovely shops (even the ones selling tourist tat actually look quite nice!), great beach, veeeeeeery long.
You can walk, cycle or rollerblade to Salou (or take a cheapish bus!), bus is about 15 mins, walking about an hour, espevially in this heat. Perhaps not best with young DC! You can hire bicycles, too, with child seats, or pulling one of those little 'pods' , very cute.
Salou is a fair bit cheesier than Cambrils, but again HUGE beach, very long and very wide, with a really nice wide promenade, wide enough for DC to run about without fear of dashing onto the road. There are lots of little play areas along the beach: swings, pirate ships to climb etc.
Tarragona is about 45mins from Vambrils by bus, 30mins from Salou, annoyingly because it actually isn't far! Buses go up and down the main beach road every 15 minutes (I think) and it's around €3 or so, one way, from Cambrils (but don't quote me on.it!). Buses leave you at the bus station.in Tarragona, pretty much the centre.
Tarragona is a Roman city (aqueduct, amphitheatre,circus, forums etc), but it's very cheap to see things and children are vety welcome to.touch, climb etc - nit destroy though! Have a look at wikipedia for Tarragona and feel free to PM me if you have any questions.