How do I explain to my children from a previous marriage that my new wife and I are having a baby?

I (male) have a 15 yr old daughter and a 12 yr old son both live with the mother and her new husband with a 15 yr old step sister. My new wife and I are planning to have a child which we haven't told my chdren yet they already stated how disappointed they would be if I where to have another child. I'm surprised that I can't find more info on this issue online. I'm looking to speak with a professional to gain any advise on this topic.