He's not challenging them, he's inviting them to play. The girls need to have toys on hand to redirect his mouth to the toys. They also need drama lessons - NO BITES - in an authoritative voice not the high pitch little girls tend to make when they're upset (my daughter is 9.)
Then if puppy continues to play inappropriately, they need to dramatically say 'I'm not playing with you anymore' and walk away.
Pupper has too much freedom. The doggie door sounds great but someone needs to be on him all the time until he learns manners. Don't expect that for atleast another 6 months.
Venus of Crooked Creek 1/26/13
Otto von Hena-C 05/23/08
Morgan Donnermond 08/04/01 to 9/28/12 Long Live the Queen!
I have duct tape and I am not afraid to use it.