I don't think I've read one where they are already a couple
But in this one the omega already has a kid and its them becoming a couple and stuff that happens after they become a couple
http://www.mangago.me/read-manga/kiraide_isasete/
this is a sequel of kedamono arashi focusing a lil more on their domestic life plus there’s another sequel after this too
http://www.mangago.me/read-manga/kedamono_arashi_kiss_me_baby/
this one’s pretty cute too
http://www.mangago.me/read-manga/tadaima_okaeri/
You want this series... http://www.mangago.me/read-manga/tadaima_okaeri/
Is there any omegaverse or mpreg where they already have a child and the story focus on their family?