When I paraphrase this sentence 'She is too strict with her kids.', I came up with this expression 'She is too rigid to her kids.'.
What is the difference between them? Please help me.
"Strict" in this context is either "strong" or "severe" (as suggested in other answers), and "rigid" is "inflexible", or "without exception", though one can also use "strict" as "inflexible", as in "the rule is strictly enforced, no exceptions are made for any reason".
Consider a parent who requires his eight year old kid to be in bed, lights-out, by 10 pm. In my country (Canada), that would not be at all strict as it's quite late. But if there was a punishment that applied at 10:01, or if no exception was ever made (e.g. for watching a meteor shower or the Northern Lights) that would be rigid, though not strict.