Unbeknownst to Lily Bowen, her grandkids are still alive...same as she, they were turned into first generation mutants. Opposite to Lily, they completely lost their prewar memories...but they haven't separated since that horrific day.
1) You can make two Supermutants called Jimmy and Becky, and you can find them together randomly while travelling in the Mojave desert. They would be non hostile, same as Jacobstown muties, and they will have generic dialogue. The idea is that when you meet them they will act no different as if you were another wastelander...even if you travel with Lily, they won't recognize her (due to their memory loss) and neither will she, because their appearance is generic (Lily's distinguishable look is a product of her prewar memories, same as Tabitha). A painful and unfortunate meeting...
Becky has that sign-gaunlet as her weapon; Jimmy can use a nail board...they are both good with melee weapons, just as her granny.
Here you have the suggested models:

Also, if you want to hide them a little, you can add two generic supermutants (not Nightkins) travelling with them, as if they are a unit.

2) You can make three Human Npc's that resemble prewar Lily and her grandkids (Laura, Jake and Bethany), travelling together in the wastes with a mercenary...the old woman could use the Brahmin-skin outfit and .32 Hunting rifle, same as Grandma Sparkles in Wilhelm's Wharf. Their prewar NPC´s are in the Fallout.ESM database, search for prewar Lily, prewar Jimmy, Prewar Becky.