Sorry your DS was being hurt at child care (I usually don't agree with the term 'bullying' when referring to toddlers, but perhaps by putting it in 'inverted commas' you feel the same - I would say another child hurt or was rough toward your child, not that he was bullying him )
I agree that you should approach the room leader, I would speak to them before a director, as they are the one who is with your child each day & knows what is going on. The director is a good resource to go to if there is a problem that the room leader is not being helpful with or is not able to resolve on their own, but I would always see the room leader first.
I would just approach it in a casual, non-accusatory way. "Hey, just wondering, do you think that there is an issue between my child and .....? What interaction have you noticed happening between them throughout the day?" Then you give the staff member the opportunity to say "oh yeah, .... is being quite rough, this is what we are doing about it", or whatever. You could then say, "I just noticed the other day, ..... was hurting my child, and I wasn't sure if it is an ongoing problem - do you mind just keeping me updated on how things are going so I know not to be worried?" Sometimes I think child care workers can take it personally, like they feel like the parent is blaming them for the other child's behaviour, so if you ask them their opinion on the matter and THEN give your opinion in a nice way, hopefully it won't feel like a confrontation for either of you. I hope it goes well!