There are two ways to do this and I'd recommend one before the other.
First way-- call the police. Tell them your mother has picked up the children and is not returning them to you. They should help out and ideally give the children back today.
Second option-- call an attorney. Tell the attorney you want to file for a writ of habeas corpus to force your mother to turn the children over to you. Ideally, this is not the preferred option because it's going to cost for an attorney's services. However, if the police won't help, this may be the way things have to go.
I think Mark's answer is basically correct and good advice. However I suspect the police will say it's a "civil matter" and not get deeply involved. But worth a try and it's free The habeas corpus is a legitimate remedy so long as you have the right to possession without a doubt. If successful it's very likely that the mother would have to pay your attorney's fees. However, it would be a slow way to rectify the injustice you believe has been perpetrated upon you. I'm thinking the temporary restraining order does more than you think it does and would like to look at it. If you will identify yourself and the mother and the county this is happening, I can look at it online