Hi, I'd like to help you with your question.
It sounds like you have done all you could to help the situation. An apology should have been enough. Now, it is up to your son and his wife to respond. And since they have not responded in a positive way, there are a few things you can try.
One, you can give it time. Let the apology stand as your response and see if time will help your son and his wife to see things differently.
Two, you can apologize again. This time, send a note or card. Tell them you made a mistake and that you wish to fix the situation in a mutually agreed upon manner.
Three, you can send the Easter gifts and hope they pass them on. The point here is that you send them, not if the kids get them. It is the parents who are causing the issue and it is them who need to see you are making the effort.
Keep in mind, you have done what you should have done. Your son and his wife are making this a power play by refusing to allow you to see the kids and by not accepting your apology. There is little you can do to change their minds if they decide to make an issue of this. Hopefully, over time, they will see their perspective on this is helping no one and they will change their minds.
I hope this has helped you,