As a gracious host, you need to put on a brave face and just do your best. Keep smiling.
Factoring in extras is the way to go when hosting a gathering. This includes having sufficient food and beverages to cater for any unexpected situation.
You may have to split party favours into smaller packages. Or simply tell the parents that while you’re happy the children came, you weren’t expecting them – so you’ll have to give them their goodie bags another day.