If your child is below 21, you have to apply separately for their settled status. They can submit an application by themselves. You can also request on your child’s behalf. If you or your spouse/civil partner are a citizen of the EU, EEA, or Switzerland, your child may still apply.
Even if your child was born in the UK, they still have the opportunity to apply unless they are officially registered as a British citizen.
You will need documentation of the relationship with your child in order to apply. For instance, a birth certificate proving you are their parent. Then, you will have to provide documentation of your child’s UK residency.
Additionally, you will need to use a legitimate ID card, like a passport, to establish your child’s identity.