The question has been asked before, and it clearly says in the rules...
"Any sim may be moved into the Legacy Family via marriage or move-in except for the following:
Sims created by the player
Sims/households downloaded and imported from the exchange"
My legacy had 4 kids in the 2nd generation and 6 in the 3rd (5 babies at once! My heir got pregnant with twins the day her triplets were born) to satisfy the "5 children" wish of the heir's husband...But now I'm worried I'm cutting it fine with them growing up before he dies and it will all be for nothing! So I'm aging up the youngest kids as soon as I get the reminder and hoping that's enough...And from now on, no more than 3 kids per generation!