I have 2 ds's aged 7 and 4. They are at school together. When DS1 was in reception he made a beeline for one particular boy who he really liked. This boy was not that keen and they had quite a difficult relationship at first. In year 1 things got worse when the other boy (who is very dominant) was physically violent towards DS1 and ended up hurting him despite my having been into school to talk to the teacher about it. I also had a conversation with his mum who is a nice woman. Now they are in year 2 it has all settled down a bit but DS1 is convinced that this boy is his best friend.
DS2 started in reception this year and has become firm friends with this boys sister (who is a complete tomboy). They play together all the time.
I have just had them here for a play date after school at the end of term and the boy was really horrible to DS1 (I told him off) and his sister is just so bossy with DS2. There's more to the story, other mothers complaining about these children, my relationship with the mum which is ok (she's really nice) and me being really anxious about DS1 with this very dominant child! What it boils down to at the end of all of this is that I really don't like the way these children treat my children and consequently don't like them much. I do the play dates because my children want them but I dread them to be honest.