To play other families, including children who have moved out, you need to go into edit town and switch the active household to whatever family you want to play. Have you tried that?
Also Sims who move out of the family home are subject to story progression, and may not even live in town anymore, meaning you won't be able to play them.
Is there anyone still living in the family home? If so, have you had them call the boys and invite them over?
Norma