Adoption advice

Note: this might not happen but I also need some advice on this for a story I am writing.

[name_m]Hi[/name_m] berries.
So it is a possibility that in the future I might end up adopting one of my best friends kids.
I was just wondering if some of the berries who have adopted could give me some advice on how to raise a child you didn’t give birth to? Like what is the right age to tell them?