It's really a statement of internal policy.
As a child on the outside thinking of going in, how would you know how many children were inside to know whether you could go in an be under the limit or potentially chastised for being that 4th child....
This "only" also points to three being the only number, not two or one.
And how would you know if they were school children or not? Maybe the implication here is school uniform. But then what about those older ones 17 with beards, casually dressed, still a child still at school......
