Does the father have to return the kids if kids lived with mother for 8 years and 3 years?

Given the statement that father recently got arrested for failing to pay child support, I am assuming there is an administrative or judicial order that he pays child support to Mother. If so, that order probably contains a finding that Mother does have custody and that Father does get a specific amount of parenting time (child support is based in part on the specific number of overnights the non-custodial parent gets with the kids every year). If this is the case, Mother should be able to take that child support order to the police and request that they enforce the parenting time provisions thereof (by giving the kids back to Mother). Mother can also file a motion to hold Father in contempt for violating the parenting time provisions and seek sanctions against Father including attorney fees. If for some reason there is no order providing for custody, parenting time and child support, Mother needs to get a Petition to Determine Custody, Parenting Time and Child Support on file in the County where she lives with the children and have Father served with the papers by the local sheriff. I would suggest that Mother contact an attorney as soon as possible.