For my birthday my stbx got me a present from the kids only - a bottle of prosecco and some chocolates - it was fine. It's his 40th too in a few weeks and I'm going to get him a bottle of whisky from them and a card. I wouldn't get him a present from me - I think it blurs the boundaries a bit.
Remember, you're teaching the kidsame the importance of thinking of others and celebrating their birthday enthusiastically. As an aside, I spotted my 7 ye old DS not singing happy birthday at a party this WE. He's not keen on other people having a birthday. He's a work in progress.
I let the children choose their dads, my exs presents. He got a milk jug last year! I do agree with a pp in the fact that his family/him should be sorting this out with the dc, my sisters certainly do for me, but it just wouldnt have occurred for my ex to do this and i only do it otherwise the dc would be very upset.