Did I handle it right ?

My SO has kids from previous relationships, but one just had a birthday Monday and so we got her, took her out. School started Thursday and so Wednesday we did last minute shopping for his 10 y.o. We're at the check out and she says, do you want kids with my daddy? I'm shocked but told her one day. She then goes to ask, will I give her another brother or sister. She is just coming around to accepting me as her father's girlfriend. Its been 2 years.