rollcall, I'm sorry that you find yourself here.
Yes, he can bring the OW around your kids at visits. Yes, it is hard to get through...especially the first few visits.
But I promise you this...OW will never replace you in their minds. Ever.
Also, if you would like an evil little silver lining to visits with OW...your ex will be brining four little kids around her...that might be fun for a few hours for an OW but the novelty will wear off fast as shiny affairyland starts looking like real life.
Deep breaths. Keep posting here. So many of us empathize and understand those feelings of sending our children over 'there.'