I would find "The Mr Thomases" much truer to a child character's voice than "The Thomases". And even if there is a plural form of Mr, I don't think the child character would know it.
I don't know how it is in USA schools but in UK schools teachers are known by their title and last name, so every teacher I ever had at school was Mr, Mrs, Miss, etc (last name) and the title was mentally processed as part of their name, not a title of respect. Also, Sir is used as well, again like it's the teacher's name. I've even heard of cases of students swearing at teachers and still addressing them as "Sir" in the same sentence, like their actual name is Sir, the kid hasn't processed that Sir is a title of respect and not a given name, and also hasn't noticed the irony of calling someone Sir and a string of expletives in the same sentence. Teachers find this hilarious (after dealing with the insolence appropriately).
Anyway, The Mr Thomases seems to me the most appropriate way to refer to the hive mind given a child POV.