I have found that, no matter how old the youngest child is (I have sometimes waited until they were 4-5 yrs old) when I next try for a baby, the very first time I try I get the "too soon" message. But then I wait 15-30 mins and try again and they usual go for it. If they argue about it, keep putting them on each other and eventually they will give in and try.
Same thing happens to me.
What I usually do is (this is going to sound real crazy and addicted, but... anyway) once they have the baby I see the time it is and add 4 hours (the baby's due time). After 4 hours I return to the game to feed them (and the baby boy or girl) and also to open the game after the birth - that way I can be sure that in about 1 hour (sometimes more than that, sometimes less) I can try new babies.
If there is one thing I don't like about the game is this thing. Sometimes the child is like 5 years old and they still say it's too soon to try another on.