I have sorted it legally but an aware they could challenge it
Whilst you can put your wishes for your children in your will, if you and your DP both died whoever wanted to have your children live with them would still go through the same family court process as any other child residence case.
Family court judges make any decisions based on what is in the best interests of the child and your wishes really aren't considered that important.
A single father that I knew did earlier this year living behind his DD.
In his will he said that under no circumstances should his DD go to live with DD's GM (DD's late mother's side) but that instead DD should go to his sister.
DD was living with the sister but the GM put in for a CAO for custody.
The sister was being quite responsible and making the DD go to school and having firm rules and the like for stability.
GM lets the DD whatever she pleases and never says no or tells her off.
Following lots of appointments, hearings and interviews including with the DD the court decided that GM should have the DD live with her.
When the issue was raised that the father had expressly said that he didn't want DD to live with the GM in his will, it was said that the wishes of someone who is dead, even the father, do not override what the court has decided are in the best interests of the child.