There should be no need for using written methods to add two 2-digit numbers. That’s the theory and by year 4 children are beginning to have to put this into practice.
Also remember that most of us do a mental calculation in a different order than doing it in writing.
For example: 27 + 35.
I start with the tens and add 20 and 30, then add the 7 and finally add the 5.
Another way is to add the tens (20 + 30) and then add the units (7 + 5) and add the two answers.
Of course you might start by adding the units, and there is nothing wrong with this, but it does not seem to be the usual way that people who are really fast with mental calculations work.
Free Y4 maths worksheets: Mentally add two 2-digit numbers (pg 1)
Free Y4 maths worksheets: Mentally add two 2-digit numbers (pg 2)