I went to a tiny primary(22 pupils down to 15 when I left) it was a positive experience, my best friend was a year younger than me, there were two boys in my year... I don't remember the falling out ever being lasting...
I would love my dd to be in a smaller school, in Scotland SNP are talking about reducing class sizes, I think early years should have small class sizes and composite classes can have good points and bad, but if you are happy with the school for your other children and not happy with your dd's teacher's "she is doing okay"... what have you got to lose?
does your dd want to go to the same school as her siblings?
my dd is about to move to another school after the holidays, she is not keen on leaving her friends but I hope the only slightly smaller school will be better for her(she is the opposite of your dd, very demanding etc... but I think small classes benefit all kinds)