We stayed just south of Salou near Cambrils.
There are regular buses and trains that go along the coast to both Tarragona and Barcelona, perhaps not what you want to do with 2 little ones.
There is a little train that runs between Cambrils and Salou stopping outside certain hotels on the way.
You can also get the glass bottom sort of Taxi boat along to different places, don't actually see much out of the bottom but kids got all excited.
Salou is much more touristy than cambrils and has lots of restaurants and brit pubs , but lots of more authentic places to eat too, good priced set menus and they seem very tolerant off kids. Cambrils has the look of a fishing village. And some lovely eating places with outdoor seating.
It will still be quiet in May. We went in April.
The weather should be plesantly warm through the day, most of the time I had a vest top on and a cardi for when it was windy down on the beach, it will still be cool in the evenings, a light jacket or jumper might be needed.
You can walk for miles along the traffic free promanade and there are bikes for hire. There are shaded play areas dotted along it.
Taxis can be hit and miss when it comes to fares, perhaps we should have asked for prices first. DH throwing in his tuppence worth on this one as he got very annoyed about the differences in price for the same return journey.
You are not far from Portaventura and it is a good day out, lots of shows and things the little ones might like and places to play. Take some of those pacamacs for the water rides as you do get soaked and the seats are all wet, cheaper to buy them here.