Ailsa,
I think the other reason that "under five" restriction exists is that children under that age naturally do things that young dogs & puppies have difficulty handling. Being too rough, unknowingly hurtful, crawling, poking, grabbing, putting (tail) in mouth and other rather rough behavior. It's sometimes easier to "just say no" than to evaluate the condition of watchfulness of each household.
One time Samma & I were at a friends' house. While we were playing cards, Samma was lying next to my feet under the table. Her baby was crawling & stumbling around in view...then she goes into the hallway. Moments later, I hear Samma give this really soft pleading whine. When I look under the table, here is baby lying completely on top of and chewing on Samma's ear, with her both her hands painfully twisting chunks of her hair.
We immediately untangled baby from Samma. Both were happy about that...but if Sam had been a much younger & inexperienced dog, she might have snapped or panicked to get free. Baby was so fast & quiet, neither her mom or I knew she had moved 20 or so feet & was under the table. Samma soon forgave & played with baby, but I still watched them both very closely. Just in case :)
Oh, Samma's a Doxie/Westie mix --very winning combination :D
toodles