Hello and thanks for using Justanswer.com! Children are amazingly resilient, but the relationship models the kids are seeing will certainly have an impact and all children benefit from consistency in their lives. Many parents agree upon a period of time in a new relationship before the kids meet a new boyfriend or girlfriend just to avoid the revolving door of adults. It can be difficult if the children get attached to the adults who are suddenly no longer in their lives. Perhaps their father can talk to their mother about setting an agreed upon time with the next boyfriend before he's introduced into their lives? In the meantime, they could certainly spend time with their mother when boyfriend isn't there.