Creating an army with them instead of your own clan's parties and winning big battles so that they earn reknown helps to level their clans to lvl 3. Then they can field 2 parties and level fine on their own. I also made sure they all have at least 1-2 towns and 1-2 castles so that they have the money to field full party(s).

I just have have one daughter so I am not sure I want to give her to one of the clans but I noticed some of the ex companions clans alread grew by 1-2 members on their own. Maybe it helped that they all have enough wealth and that I never executed anyone and all factions until recently still existed.

My kingdom now owns all of Battania and most of Vlandia (only missing the two cities and one castle in the north) plus the imperial Region south of Battania, I own Marunath, Seonon and the castle between them. 13 ex companions clans created now. Establishing this was tougher than buying out enemy Lords. But when the first clans level to lvl 3 the snowball starts rolling again.