First of all, Vito's grandchildren has been mentioned and has also been capshot in the movies. It's not significant but it's fun to know for us, voyeurs.

I would think that Sonny has 4 children and 1 bastard, which is Vincent. I say this because in part I, he has a son named Frank who read the 'get well soon' card to vito. Then there's the twins and a small baby that his wife held when Sonny received the phone call from Solozzo.

As for Connie, I would think that she has more than 1 children, because Michael mentioned about her eldest boy being caught for theft in Reno in part II.

These are just my opinions.