Thank you Jamie I will try that next time I play my game but I think that when an additional home is purchased and its shown in purple in map view it seems like the only options are to buy on this lot, build on this lot or move and then everyone moves.
But thank you for responding I'll give it a shot! 
With the latest patch/expansion property ownership has changed. Since your family owns the property already I don't think the game will let you move out the children to that house (since only the options you mentioned above work on an owned private property).
If you want them to live in that house sell it using the real estate menu then immediately have one of the children use the real estate - move option on their phone. You should be able to click on a button that says 'To House' or something similar and select the house from a map of the neighbourhood. Select the sims you want to move out, make sure your main family still has the Active Family button checked under their name, and that should be all you need to do.
Hopefully this works for you!
