Ex Husband Passed Away Leaving 2 Minor Children

My ex-husband passed away a few weeks ago. We have 2 minor children, son 14, daughter 8. He did not have a will or beneficiaries, so his 401(k) and life insurance are automatically going to the children. Since they are minors, I have to figure this all out. I basically need to get court papers stating I am their guardian and that I can accept property on their behalf. We did have 50/50 custody, an agreement we came up with ourselves and had legal papers notarized, would that be enough? I tried calling the court house and spoke with every office it felt but ultimately they told me to hire a lawyer, is that necessary? I am in the state of PA if that matters. Any advised would be helpful!