I have never borne children, but my vagina is larger and looser in my fifties than earlier in life, but I don't think its related to intercourse so much as simply growing up and growing older! Ahh the joys of being "seasoned". Lol
As you age, or have children this will happen anyway. Muscles lose their tightness etc and so it's bound to happen. Doing exercizes like kegel ones should help, but ultimately it will happen eventually. No ones vagina is ever the same in their 30 onwards as it was in their 20s, that's just what happens.
Possibly... I have a very demanding Fiance and I have asked my Dr. this same question before, he suggested doing kegal excercises daily.