It is hinted in the lively opening moments of Mill Mountain Theatre’s “Into the Woods Jr.” that the audience is in for a happy ride. And the talented young performers — all members of the Mill Mountain Theatre Conservatory for kids in grades six through 12 — definitely deliver. The hourlong show is an offshoot of the Tony-winning musical by Stephen Sondheim and James Lapine, but with important differences: It has been condensed and stripped of the murder, infidelity and other