I have had three kids and I have been told that I am tight so having kids doesn't make the difference.
Now in the past, I have had a time when I was in lack of a better term "s l u t t y" and was having sex all the time and I became a little looser but since then I have tightened back up.
Having kids doesn't effect this only having sex with larger men or all the time will cause that.
maybe it's you. perhaps she's had bigger than you for a long time and that's what she's used to. but like i said giving birth to a child (even if it's more then one) won't affect that. a woman can stretch then go back to normal. so maybe that is her normal.
having kids should not affect that. i had twins 8 months ago and my husband has had no issues with that. or complaints. he actually said it's exactly the same as before i had the kids. was this the first time the 2 of you had sex?