My vote would be for SYD-SFO/LAX-EWR (see above for notes comparing SFO and LAX, but I would lean in favor of LAX). I think UA is a good choice here for having a 1-stop routing that's lie-flat all the way and the 787 cabin layout is friendly for families. If you really care about service quality to the exclusion of schedule convenience, CX or JL are probably better choices.