As far as I know your ex can not remove the children as beneficiaries to his life insurance. If he was unable to maintain the premiums and cancel the policy then that would be another issue.
The premiums are based on a number of things, his age, amount insured etc and has nothing whatever to do with the beneficiaries that are listed. Has he given a reason for his changes?

My daughter too has had to make do with less than 50% of what her father should pay for maintenance and he has cancelled all his policies and spent his pension in order to "get back at me". He knows that I will not allow our daughter to go without a good education whether he pays or not, but it does mean that I go without many things and cannot live in the area I would like to in order for me to be able to afford the school that my daughter goes to.

I think that if you get in touch with the Maintenance court or Social Services they would be able to advise you on the legalities of your ex's choices but what benefit the knowledge would be in the long run is debatable.

I know that Grandparents can be sued for maintenance where the parent is not contributing, but whether this extends to his sibling is not clear. Again Social Services should be able to help.

I know how frustrating this all is, but there comes a time when for our own sanity, we have to pick our fights. Perhaps this is the one that is best left alone.

Loads of luck. xx