I would say that the relationships the kids have between you and your H should be separate from the issues your having as a couple. Saying that, I think it would be good for them to give their Dad a card, wish him happy birthday etc. It may not hurt for you to at least wish him a happy birthday as well but maybe not make a big deal about it.
Since your situation is so new and it seems your H is racing toward divorce I can understand you may have a feeling of panic about the situation. I know my W had been pushing very hard for a D, wanting to sell off all of our assets etc. That hurt a lot but I think she's realized that if she does go ahead with a D it will take a long time and she's backed off for now. Where we live she cannot even file for a D for 1 year (she's been gone for 4.5 months so she has a while to go yet). A lot can change in a situation given time so just because your H is showing every sign he wants to D does not mean he will not change his mind.
I would say make sure you and the kids are protected and continue working on making yourself the best you can be. In time, your H will notice the changes and it may at least cause him to rethink his actions.
Me-40,W-37 D7, D5, S3 Separated Oct 3/2013 T 11 YRS M 7 YRS