Before continuing with multiplication it might be wise to review place value.
Because we use a Base10 number system, multiplying by 10 or 100 (or any multiple of 10) is fairly easy.
As the picture below shows, each place has a value 10 times that of the place immediately to the right
If necessary, review place value before proceeding with the multiplication methods below.
Multiply integers by 10 by moving the digits one place to the left and adding one zero.  

Multiply integers by 100 by moving the digits two places to the left and adding two zeros.  

Quick Way: count the zeros and put them to right of the integer being
multiplied.
e.g. 34 x 100 = 3400
Caution: Highlight to your children that multiplying by 10 or 100 can be more than just adding zeros as this can lead to problems when multiplying decimals. e.g. where calculating 1.4 x 10 = 1.40 could incorrectly result. Stress the change in place value of each numeral as it moves to the left.
Try this multiplication worksheet generator. You can select to multiply by multiples of 10 or multiples of 100 and get limitless questions to print out and practice with.