If the child is very young, they will have no concept of what is going on. The child knows the foster carers and will enjoy a new experience with them, which presumably will just be blowing out some candles and opening some cards and presents.
Our children had a leaving party at the fcs which doesn't sound that different to a birthday party. They were nearly 2 and 3 and neither remembers it.
What IS more likely to upset them imo is doing it with you, basically strangers.
My advice is to do nothing big with your child in the first few weeks in case it gets linked in their minds to sorrow or worry.
My younger daughter used to throw a complete tantrum on a particular stretch of road. It took me years to realise this was the way we had brought her home - we rarely went that way, but when we did it triggered her.
We also, on advice from post adoption therapy, painted our front door a different colour - when we came back from holidays, she would again be very angry and upset.
Our other daughter wasn't affected.
Imo you are safest to let this birthday go.