Adding and subtracting involving decimals is certainly best tackled after your child has grasped the concept of tenths and hundredths. You will find more on place value with tenths and hundredths here.
Assuming this concept is understood, then the essential step of lining up the decimal points when adding or subtracting should be more easily followed.
Be sure the decimal point is included in the answer
You can review with your child how, when adding or subtracting whole numbers, zeros can be added to the left of a number without affecting its value.
The same logic can be applied to the tenths, hundredths, and thousandths using the base 10 blocks as a visual reference.
This will help when adding and subtracting numbers with different numbers of digits.
The example below shows a common mistake that is made when adding and subtracting with decimals.
Lining up the decimal point when doing vertical addition or subtraction makes the calculation much easier. Adding zeros to the right of the decimal numbers may also be needed. When adding with decimals and whole numbers students may also have to add a decimal point on the right of the whole number. Finally, estimating the answer and comparing it with the calculated answer will help avoid errors that do not make sense.