T.N.
Wait, you're only 26?! I think you were born 50, Ephie!
If they will continue to see BOTH your mom and stepdad, I see no reason to say anything at all.
Going to his house? 'Grandpa's got a new place now, wanna go see his new house?'
'Where's Grandma?'
''She still lives at the old house!'
Kids will be happy with what you give them. It's only US that hates change and thinks it's weird.
:)