When can a child with ILR but born outside UK apply for naturalization?

If a child is born outside the UK and both the child and parents now have Indefinite Leave to Remain (ILR), when would the child be eligible for naturalization? Can we apply for naturalization immediately for our child, or do we need to wait at least 12 months? The child has been in the UK for the past five years.

Thank you.

The child can apply to REGISTER as British once either parent is also applying for citizenship (or is already British) and the other parent holds ILR.