It's really not normal to turn up to a party without an invite, whatever age! It's rude. As soon as I realised there were that many extra children in that situation I would have told the parents that due to numbers they would have to keep the siblings by them at the side.
I had a similar situation at a soft play party, despite giving the venue a list of the children invited. Parents had to sign in at the door with the name of their child, as we didn't have exclusive use this was to ensure that extra children weren't joining in the party for free.
What actually happened was that parents signed their child in and their sibling, the assistant totted up the number of extra children and then charged me per head for them, at normal admission rates rather than party rates, they also provided extra food and charged me per extra head for that too! There were 15 siblings and 30 original party children who I had already paid for. I had not agreed to this and I refused to pay, the bill was over £150.
I told the venue that I was not paying under any circumstances, I'd given them a list of names, they were given names when the parents signed in and they should have charged for children not on the
Iist. If they weren't not prepared to take the hit then they should have stated 'no siblings' on their invitations which they produced.
I got in touch with all of the parents on the class Facebook page, I explained that the venue had charged me for all siblings and food. I asked them if they'd please contact venue and pay for their child's admission and food who was not on the invitation and, as much as I didn't want to leave anyone out, siblings had not been factored in to the cost.
Most parents were fine, they apologised and then phoned the venue to pay. I then contacted the venue the week after and paid the remaining balance which was about £30. There wasn't any awkwardness afterwards and I noticed on most party invites afterwards it stated 'no siblings'.
As anticipated, siblings really took away from the party. People shouldn't be so cheeky and I'd definitely have spoken to the parents at the time.