Ball boy, as two words, is in quite a few dictionaries. Of the online dictionaries I looked at, Collins and Wiktionary gave the one-word
ballboy as a variant spelling. Oddly enough, the
online Oxford shows only the two-word form, although every one of the dozen or so usage examples given has it as a single word.
News reports retrieved via the News on the Web corpus use both
ball boy and
ballboy, though the former is more frequently used.
Ballboy will be allowed in future, but given that it's a less frequently used variant form, I won't make it common.
It turns out that
ballgirl is already accepted in Chi.
Ballkid seems to be used mainly in Australia and New Zealand:
It is listed in the Macquarie Dictionary, as a single word. I'll add it as a rare word also.