What to say to a child

Tanya • Im 27 i am engaged and have a lil girl of 7 and pregnant with #2
Hi everyone my fiance and i have been trying for a baby for 8 months now it will be his first but my second i already have a 7 year old and its no secret in this house that mummy would like another baby obviously katy foes not know the ins and outs of how to make a baby she assumed when a woman wantted one she just got one in her tummy but as she has heard my fiance and i talk about it shes now asking why im not getting one what can i tell her to ease her mind.....shes desperate for a lil bro or sis lol but far too impatient