Leigh did explain what the flip flops are, so if he said "I'll see you cats in the flip flops", that would mean "I will see you wearing beach footwear". However, he told the kids "I will see you cats on the flip flop" which I believe means simply later. On the flip side would mean after something, on the other side, so maybe on the flip flop means tomorrow or after a break, I can't remember if something specific happens in the movie at that point.