British English / a strange way of using neither nor [duplicate]

Solution 1:

Standard Englishes don't allow negative polarity words (none, nowhere, never, neither) to be used with explicit negation (not, isn't).

So your sentence is not grammatical in any standard English. The negative may either be in the verb:

This game isn't good either for children or for adults.

or in the complement:

This game is good neither for children nor for adults.

but not both.

[Many people throughout the English-speaking world speak varieties which do allow so-called "double negatives", and your example would be grammatical in one of those varieties. But not in any Standard English]