If every parent of every person at the party is aware that your parents are allowing alcohol to be drunk on their premises by under 18s and if every parent of every child that is likely to be drinking said alcohol in your house gives their permission for their child to drink said alcohol, then your parents will avoid prosecution, but any parent that does not give such permission and subsequently a child of the none-consentin parent(s) is susequently found to have become intoxicated from the consumption of alcohol on your parents premises after such sought after permission was declined or refusesd, your parents can be prosecuted.