I have done that and just used the pink floating heart (under electronics) of the insim and made them all mutual best friends. Then you have to go and make the two parents again and only them mutual love with the pink floating heart. You may need to use the red floating heart to make their LTR and STR at its highest level too. That should fix it for you as it did for me. My kids were bawling their eyes out anytime they saw their parents kiss. Maxis is very closed minded when it comes to love. There is a mod that saves the LTR's and STR's so they do not decrease after they are at the highest point.
A lot of hacks from MATYSquinge has a hack that keeps relationships from decaying and that can be found here:
no relationship decay hacks