At a park 2 hours won't be enough time...lol. They'll want to run amok and play. If you don't do too much structure and let them play you can do any time frame easily.
I'd do it like this.
Open the party with the craft. This way if one or two of them need more time it won't be a big rush.
Then I'd let them go play until everyone is ready for cake. Do the happy birthday song then hand out the plates with the cake on it.
Then open gifts. Now it's all basically done. If you serve food you add all that cost and mess. I wouldn't serve any food other than cake and maybe ice cream.
Then I'd let them go run amok and let the parents round them up when they're ready to home. This way the party part is over but the kids are still together and having a blast playing.
If you have the party from 10-11:30am you can do the cake around 10:45 to 11, not a bad time for a morning snack. The parents will round their kids up to go home for lunch or out to eat. They'll have run off the cake and will be ready for their afternoon naps.