To want to attend my grandchilds christening

What is it about the event that is important to you? If it is the religious bit then you are a bit stuck but if it is the family-gathering bit then can you convince them to continue the celebration in the evening or have another one at the weekend - you can't be the only ones to have trouble getting time off work.
What are son's DP's family doing, are they having the same problem?

Can you put the ball back in their court: say that you really want to share this special moment with their PFB (and other such fawning stuff) and what do they suggest is the best solution.

Thankyou supernana for your kind words, what you say makes sense, I will have to just bite the bullet.

Oh dear! I feel so sad for you. It is natural for you to wish to attend such a special occasion. I do feel that you are being treated badly and have every right to feel deeply hurt. Making a fuss isn't going to achieve anything, I fear, as minds have been made up. It will so difficult for you to bite the bullet...but, I would choose a beautiful card, insert a loving message (no recriminations) and wait...I wish you well.