Just don't ask unless you are absolutely stuck and then you could just decline, offer to drop and go or ask another staying parent to keep an eye.
Along with tens of other ways already mentioned that put the host out- entertainers have often a limit on numbers, whether is 12 or 30, and at some point some contracts terme require two entertainers or increase in fees if you go even one above the number.
Some places will just do 6 and that's It, etc. It's not the hosts obligation to run it by you and to explain it all to you.
With the poor manners in society, while many don't bother to reply to RSVP, and are unable to commit, you don't know how many will come till last minute, so if invited tbe whole class , and you are asking me about a spare kid, I'm above and that puts me in a shitty position of having two entertainers because of your kid in case all invited so come- the fact I won't have confirmed till last minute.
Same with hall 30 or 10 or 5 doesn't matter, they may be the limit for the hall and the parents.
You just don't know what arrangement the host has for their party and what contracts and suppliers in place so just accept that only the invitee is invited and don't push your way in putting the host in the awkward position of either having to apologise to you or explain what terms they booked.
Same with softplay. Some require you supervise the kid in the party, not the sibling in the corner if you do manage to keep them away.
It's rude to put the host in this position just because you don't understand how parties work. Just accept that they invited whom they invited and they had their reason. You can have a big and long party for all your kid's friends and their siblings and parents or carerss and grandparents including neighbours and the whole year at school. Just wait till it's your turn and I'm sure that's what you'll do, the open minded gregarious bringers of uninvited siblings.