If they can get it approved from social services, yes.
I like to give my foster kids the same life experiences that I'm able to give my own kids and that includes going on vacations. Some abroad, most out of state. Most of the kids have never been able to do things like this before.
Parents wishes are considered but don't necessarily affect the decision of social services, either. I've been doing foster care for 20 years and none of my vacation requests for the kids have been denied. Not even last minute ones that were planned before the kids were placed with me.
I've had a harder time with field trips that were out of state, but I manage to get the okay on those too. One of my fosters went to Washington DC on a class trip last week and I had to do all sorts of unnecessary BS for them to have the opportunity to go.
I don't see an issue with the kids going on a vacation/holiday? I'm leaving town for a few days/weeks, I'm not skipping town.
As far as official ruling on custody.. there has been a ruling... the state/government has custody of the baby, not the biological parents which is why foster parents really only need the approval of social services.