My best friend passed away in Sept this year. About two years ago she had me sign a form making me the sole beneficiary on her pension she drew off her exes work. Her grown kids have told me they are getting it not me. They were estranged from her for years. What should I do in this matter?

A beneficiary can be changed at any time while someone is still alive, or it's possible that she never filed the form, in which case the children most likely would be beneficiaries.
If you didn't keep a copy of the form, you may have a tough time tracking down the name of the company that pays the pension. Start with the place where her ex worked if you know that. If not, and the children won't help, you will need to hire a lawyer.

Also. Normally a pension ends when a person dies, unless there was a survivorship option.