Documents required for British stepchildren?

[Jem0811]

Hi
My son-in-law is due to apply for FLR as a partner to his wife, my daughter. They have 2 children together, both British citizens. However, my daughter has children to a previous marriage who live with them. Does he need to provide the birth certificates and passports for his stepchildren also? The form makes it sound like he does, but it will be difficult as eldest child needs her passport for a school trip. Thank you for any help you can offer.