Developmentally, children will write numbers backwards until they are
8 years old---after that it could be a sign of dyslexia. Many
children really cannot see any difference, especially in kindergarten.
Most children after January of first grade can see the difference and
can be taught to look for and correct their backwards numbers.
The way you deal with backward letters and numbers can make a huge
difference. Please do not make a 'big' deal of these. Speak to
children quietly/privately if you must and let them know, but be
prepared that they may not be able to see the difference yet. In
kindergarten, we need to keep encouraging them to write and if you are
too picky,you can shut some kids right down and they will stop trying
to write at all because they are too worried that they will do it
wrong. Your attitude should always be, "when you are big enough,
you'll be able to do it".