Worried about my baby being a boy

I already have a 6 yr old boy and a 3 yr old girl. When I found out I was pregnant, I just wanted a healthy baby! Well I told my kids they are going to have a little brother or sister. Both my kids are very adamant that this baby has to be a girl. My son especially. I think he wants to be the only boy. Now I'm worried they'll be disappointed and reject baby #3 of its boy. Would they get over it you think? I just want my kids to be excited and happy about this new addition