In response, headmaster praised me and hinted that some parents are overwrought. Also that if it didn't happen small girl should be monitored for signs of sexual interference.
I got a second phone call from other mother where she reiterated how calm she was...not at all overwrought...no! not her...no, super calm. CALM! CALM! CALM! Hear how calm I am!!!! Then she told me about how she coaches her daughter on all the family members who are not allowed to touch her vagina...seriously. At which point I began to think perhaps small boy was in the clear after all. There's more to the story (unfortunately) and through it all our child comes out smelling of roses and the little girl is getting more and more stuff flung at her, which I feel is borderline victim blaming (this whole thing has been surprisingly triggering for me).
It's now at the stage that the least worst option is that our child licked their child's genitals. And, honestly, if that happened, fine. All part of life's rich pageant. But it did alert me to the fact that we do need to enforce personal boundaries with small boy (who, for instance, loves to hold my breasts).