In terms of a general rule, the ROC tax authorities are interested in seeing that you are supporting yourself with an adequate income, and that you are paying taxes on that income.
Hence, to be safe and to meet the minimum requirements, you should declare a reasonable level of income and pay tax on it.
The alternative is that some day in the future the tax authorities will arbitrarily decide that you owe “back taxes”, and they will assess you an amount of tax due. I have heard of this happening many times. If such “arguments” or “differences of opinion” do actually occur, indeed it is the taxpayer who usually loses.
To my knowledge, the Ministry of Finance has never ruled that salary income from kindergartens is tax-free.
For all technical tax questions on your own individual situation, I strongly suggest that you visit your local tax office in the ROC. With a few minor exceptions, I have found that the advice and counseling they offer is quite good.