Hi L.,
My girls are just about the same age as your boys. I have run into the same problem especially for my youngest as well.
They attend a great Nursery School in Seekonk, MA, it is right on the Pawtucket line. From Providence, it would take you about 20 minutes to get there( I live on Providence side of Pawtucket).
It is called Knotty Pine Nursery School. It has been in business since 1949. The director Penny Smith, her mom A. Ainsworth started the preschool. They have a 5 day a week program if needed, but you can pick from 2-5 days, what ever works. She offers morning, afternoon, and allday programs. It is reasonably priced. Discounts for siblings are offered. The boys would see each other at various times while there.
FYI, children have to be potty trained to attend. I know that she is taking applications for Fall now. You can go and observe etc. The children learn a lot in a warm setting. It isn't fancy, but I have never seen such a happy, well adjusted, polite group of preschoolers. I myself attend as well as my cousins. My oldest just turned 4 and is almost finished her first year there. She loves it!
Good Luck, I hope this helps.