A question about "spend on"

The following sentence is a sentence in my composition, and I think it is reasonable to write a sentence in this way. However, a grammar checker has said that the preposition word is unnecessary. Is it a false negative check result?

if children spend much time on watching interesting TV shows

This is yet another case where 'grammar'-checkers unnecessarily confuse inexperienced writers by treating as a hard and fast rule something that is merely stylistic advice. It is true that on is unnecessary, before an -ing form of a verb, when one speaks of spending time. Because it is unnecessary, it simplifies one's writing if it is omitted, and that's why one may be advised to do so. However, one does not violate any rules of grammar if one nevertheless uses it.

The explanation of why such sentences are grammatically correct both with and without on has to do with the well-known fact that, in English, the -ing form of a verb can function as either a gerund or a participle. If, in the OP's example, one thinks of watching as a gerund, then on is required, just as it would be if one had an ordinary noun in that place. On the other hand, if one thinks of it as a participle, then using on would be a mistake. However, although these two ways of looking at watching in this case are very different for the purposes of grammatical analysis, that doesn't make much difference to an ordinary user of the language; for the purposes of advising such a user, one can thus say simply that, in such sentences, on is unnecessary, and that it may therefore be preferable to omit it, but that using it is not incorrect.

In some cases there may, however, be stylistic reasons for using it, which the software cannot take into account. In particular, because on is regularly used in money-related contexts, one may choose to use it for time (and so deliberately ignore the advice), when one wishes to get one's readers to think of budgeting time as analogous to budgeting money (e.g. 'if you spend some time on maintaining the machine now, you will save time in the long run').