So... no interview with the girl? Glossing over the fact that he's "rough-housed" this same girl before? It sounds like this kid has a pattern of violent unwanted physical advances.
Didn't say prison, or ruining his life or anything. But a suspension seems appropriate. And while a euphemism may be warranted for someone his age, it is technically sexual assault.
Enough to know that kissing someone who doesn't want to be kissed is wrong, and rough-housing with someone who doesn't want to be rough-housed with is wrong. It's not complicated...
You're assuming she didn't want to be kissed (we don't have the girl's perspective on this; apparently they're "boyfriend and girlfriend", which I don't think are valid terms with six-year-olds anyway...).
Expelling imho is very extreme in this case, there's other ways to communicate to the boy/class that it wasn't appropriate without making a way-to-public statement about it.