What to do if my kids and I have been subpoenaed to court to testify against my girlfriend for violating a OOP, but I don't want my kids testify against their mom?

The prosecutor/courts have the subpoena power to order your children to testify, even against your wishes as therei exists no immunity privileges between children and their parents to prevent them from testifying against their parents. If the children themselves are reluctant to testify, then they may not be called to testify, but if they have been subpoenaed then they must attend the hearing and testify if called to do so.