As you said earlier, you found them. Yay
Now , do the following:
1. Put the Lego in different boxes, leave the old boxes behind so your parents don't get suspicious.
2. If you live in your own place, stash the boxes in your house.
3. if they find out they are gone, deny everything
Another plan would be to ..
1. Find something your parents had when they were
young.
2. Send a threatning phone call to sell
it.
3. Ransom for your legos and for them to
stop loving with your stuff.
Edit: Another idea : buy it back
and this plan is 100% serious
this is probably the only ways you'll get them back