Explain to your child that multiplying decimals is almost the same as multiplying with whole numbers.
The only difference is that the number of decimal places in the answer must be the same as the total number of decimal places in the question.
You can use the idea of hops instead of decimal places.
When tutoring your children on decimals be sure to explain the real-life relevance of multiplication with decimals. Money is usually an idea that children can identify with. e.g. if you get $5.50 every week for 8 weeks, how much will you have at the end of the eight weeks?
The example below highlights are fairly common mistake that students make when multiplying decimals.
"Using the wrong rule" type errors are best dealt with by discussing the underlying principles. This will require a solid understanding of place value and knowing that moving the decimal point one place is equivalent to multiplying or dividing by 10 depending on the direction of movement.
This error also highlights the benefits of estimating the answer to check whether the calculated answer is reasonable.