Arenal is probably the busiest of the resorts you have listed. I have stayed at Arepar twice, when DS was 4 and 6 respectively. We went in mid-June the first time, then end of July. Arepar are great apartments - nice pools, spacious 2 bed apartments, well-equipped, clean - you won't have any problems there.
There are lots of restaurants and bars in Arenal, I don't know who has told you otherwise but I would totally disagree with them. They range from Sa Gavina (head-waiter called Fernando will make a big fuss of your DC's who will then badger you to go back every night) - nice Spanish food but quite expensive, to the Good Bridge Cafe - more English, good value. There is a Chinese, several more Spanish restaurants, an Irish bar, and lots of shops in the Romani centre.
Arenal served us very well when DS when was young and we were new to holidaying abroad with a child. The beach is stunning, shallow water ideal for children, there are go-karts and bouncy castle things at Goody Goody on the beach, pedaloes, kayaks and more restaurants down there too.
Santo Tomas is nice - but is much smaller than Arenal, but I think you would enjoy it just as much.
I have only visited Cala Gladana once. I didn't like it, but everyone else I know who has been there does, it is very like Arenal, only even prettier.
Whichever you choose, you will have great fun and everything should be open for the season when you go in late May (season starts 1st May).
Enjoy! I am heading back in July for the 10th year running.