Yes, you need spare party bags even just a balloon, cake type thing. Or just a bag with cake for older children?
We had this last year, did a whole class party for the 4 year old, just music, party games and snacks in a hall. We said parents were welcome to bring siblings but please let me know for catering, and had a couple of babies and toddlers turn up I expected plus 13 older children ageing 9-12 only 2 of which RSVP.
I had done the party bags for expected kids plus expected siblings and another couple of spares. I did not have 11 extra spares on top. I admit I also didn't expect some of the 10 and 11 year olds to be pushing all the 4 year olds out of the way to win running races and to win pass the parcel.
I expected parents to stay at that age, so had plenty of food and prizes like sweeties,
hair clasps, cars etc in a dip bucket but still had to scramble round to make extra gift bags as all the older children expected them too.
We also had a child arrive who was invited but hasn't RSVP, they brought a much older sibling with additional needs and the parent asked us multiple times to put the music off, stop it being too noisy for this additional child, and stop some of the party games they became triggered by. We did offer multiple times for them to leave the younger child at the party, and come back at the end. We offered to bring the younger child outside at the end to thier car if it made it easier. We also offered to let them go home early with party bags x 2. They insisted on all staying but they also kept asking for allowances such as being able to blow out the cake candles on the birthday cake, before the birthday child, pass the parcel upset them so they asked us to stop playing it, there was food ont the buffer the older child was not allowed and they asked us to remove it...
If we had actually invited them or know they were coming I would have really tried to make it better for them, with information in advance. At the time, there was little I could do without spoiling the party for 30 x 4 year olds we were expecting. We did have some help from a couple of family members, but it wasn't really possible to rearrange the whole party round this one 12 year old child and thier needs when we weren't even expecting them or thier younger sibling.
It did make me reconsider doing this type of party for my other child .