Your request isn’t unreasonable, but then again neither is your husband’s desire. There are several factors that impinge on the situation that is normal and not likely to change any time soon. First, the co-occurrence of family events is likely to create conflicts in time and commitment for a modern family. Your mother-in-law’s birthday celebration is just one of many things that will put a demand on the collective family time.
Secondly is the cost factor. Flying four people to a destination rather than one is a very real concern. While flying the family out to have birthday celebrations at your place would be a splendid idea if the family lived close enough, but it seems like the surprise with your mother-in-law and the other family members coming to join her at her place seems the least intrusive for everyone — and your husband sacrificing being at his children’s events also seems like the least disturbing. In summary, him going has the smallest amount of disturbance time-wise, financially, and disruption wise in the larger family system.
That having been said, this is a huge deal for him to leave when he is missing his children’s activities and this is a joint responsibility for the two of you to deal with. Your husband abdicating his responsibility for talking to his children and working with you on the arrangements, and you demanding he alone decide which event you would go to and hire someone to attend the other are both counterproductive. The goal is to work together so that the family system can stay as balanced as possible with everyone’s needs getting met to the best of the family’s emotional, physical, and financial resources.
Grandma having a birthday doesn’t need to be the cause of greater fractionating in your family. I’ll encourage you to talk with your husband about the best way to keep your children engaged in their activities while they too can honor grandma’s birthday and not feel like dad has chosen her over them, you have alienated yourself from their father, and a family surprise celebration doesn’t turn into a source of anger. I’ll encourage you to have this talk sooner rather than later, and if you need help you can contact someone from the find help section at the top of the page for a consultation — or get help from a licensed marriage and family therapist here.