To have them move out, choose "Move" from the phone like any other time you have moved houses. Then, you can choose which ones are to move out. You can choose only the grown-up kids, and either choose a house for them (this will cost you money!) or choose to 'evict' them, which means the game will place them in a random house in your town and you don't have to pay.
I always play multi-generational games as well. I let my chosen child, the heir, to stay at the childhood home with their parents, so she can take over when the parents are elder. Other children get new homes around town and eventually have their own families due story progression.
You can only control one household at a time, so after you move the kids out, they will be like townie households. You can visit but can't control them. Of course, if you change your mind you can invite them to live with you again. They and their future offspring will show up as relatives on your sims family tree.