something wrong here,you should go back to US consulate
as soon as possible,explain your situation (baby in home) to VO,( BTW ,you should ask twice entries when you took the interview ),I belive VO will change it.I
had same experience about 8 years ago,& when I went back to the US consulate,the VO cancelled old one,and
gave a new one that was mutiple.
Otherwise,you might go to USCIS office at Vancouver
International Airport,talk with US immigration officers
there try to get a parole,an I-94 card that is issued
by USCIS,it will allow you tranfer in USA to stay only
one day to pack up baby,so you may keep the single visa for your coming back from China.You may need your
hu*****and or else person to carry baby to airport in USA
then hand to you,if your parole limit you leave the airport.Good luck.