Sounds like you did well with this situation. I suggest that you get a book written for young children about good touch, bad touch. I think that is even the name of the book I've read with my daughter and grandchildren. Look thru it at first so that you can pick and choose parts that are appropriate for your daughter. It's illustrated in cartoon like characters and makes brief easily understood statements. The book also includes a pamphlet with suggestions for parents while talking with their children.
I also suggest that the boy's mother also read this and/or other books about body parts with him. I think it is OK to wait and see where this goes with him before seeking professional help. Since he's 7 perhaps she can have a sense of where he's at with having been molested and the exploration he's doing now by having a friendly talk with him. Exploring at his age is very normal. It's unusual for him to kiss her private parts but not unheard of in the process of exploring. I suggest he may know about the kissing if he was kissed in this way or was asked to kiss or if he's seen anyone doing this. Say, in a movie his parents were watching when they thought he was asleep or if he burst into their room. It happens.
Since I don't know what happened to him when he was molested I can only guess about the result. I think it's very possible because of his young age and if he was not shamed about it that he is just fine and not acting out in this way as a result of his prior experience. If the cousin was also young it could have been exploring too.
Young children are not usually traumatized by being molested a few times in a non-violent way. They are traumatized when the parents punish them or say things that cause them to feel shame. It's the after effects that do the most harm.
Rarely does a 3 yo child have to testify in court. You don't say whether or not this went to court. If more happened after the molestation was stopped, such as being told that the offender was an evil person who will go to hell sort of over reaction in front of the child then he may need some counseling now.
If his mother will feel better about this if she has him evaluated then she should do that. She should be sure to find a reputable, experienced and skilled person to talk with her son so that he isn't psychologically harmed by the interview. She can call the American Medical Association, CSD and/or the police departments sexual offender unit to ask for names of providers. She doesn't have to give her name.
Based only on what you've written and my experience as a sex offender investigator my guess is that this is a case of normal exploration and not an indication that he will become an offender. I'm especially glad to know that the acts were seemingly consensual. Most often I've seen young offenders, usually approaching the teens in age, who've been molested as acting out in angry ways.
As another mother suggested it's likely that your daughter will forget that this happened. I also suggest that the boy doesn't remember his past experience unless someone made a big deal out of it.