Even if child doesn’t, it helps to have a land yard or something on him/her that holds important contact details in case of any emergency.

Frankly speaking I can't remember whether they learned it in 5 years or not

I will tell them to remember it by hard. For emergency

No, I don't think he used to remember when he was 5.

He has been taught to memorise my mobile number

Yes I make sure they memorise our numbers

My child does remember my number

Not really, modern problems

Not yet , should teach him

But its also on speed dial