A simple way to assess which number is greater is to move the decimal point to the right as far as you can in both numbers (as long as the decimal is moved an equal number of places in each).
Doing this for 0.100 would give you 100. Doing this for .101 would give you 101. It is easy for a student to tell which of those numbers is the larger one, and the reason why is exactly the same. If all you are trying to do is teach them how to put them in order, that is one easy and sure fire way they can use every time.
I also have found it is easier when students say the whole decimal out loud. One hundred thousandths, and one hundred one thousandths. What usually confuses them is the visual, and saying it out loud gives them another way to see it.
0.100 ot 0.101
Let us rewrite:
0.100 = 100/1000
0.101 = 101/1000
Comparind both roots we note that 101/1000 > 100/1000
Because 101 > 100 and the denominators are equal.
We need to determine which is larger : 0.100 or 0.101
The first number is simply .1
The second number can be thought of as .1 + .001
Taken that way, the second number is obviously larger. In general, the more numbers (and/or larger numbers) are in a decimal place, the larger the number.
o.100 or 0.101
0.101 is greater because it has 0.001 more than 0.100