There are a couple of techniques which help with being able to double multiples of 5 quickly ‘in your head’.
Firstly any 2-digit multiple of 5 which is also a multiple of ten (eg 60) is straightforward as only the tens digit needs to be considered.
It is harder to double numbers such as 65. Probably the best way is to double the 60, making 120 and then adding 10 (which is double 5) to make 170. Some children prefer to double the 5 first, but as this always gives us 10 I prefer to do the tens part first, knowing that I have just got to add 10 to the answer.
This is probably most suited to year 3, although many older children will benefit from the practice.