How do I get my kids back?

Assuming there is a court order setting out these custody and access arrangements, you would seek to change that order by way of a motion to change. You would need to set out some material change in circumstances, for which you can obtain further clarification from speaking with a lawyer of your choosing. In your motion to change you would apply to have custody and access of the children. It is quite possible that both you and your father have experienced a change in circumstances since he was granted custody of the two older children. These situations are not uncommon, and the existing court order was not intended to be final meaning forever without variation, because the legislation allows for the final order to be changed in the event that the circumstances warrant it being changed.