Generally speaking, the amount at line 305 cannot be split other than in the year of separation, and I think even this would be tricky because it would likely be an override in Profile which will probably trigger a paper return. I never tried it. In a situation like this, it's best that both parties be made aware of what the other is claiming because one thing I do know for certain: If both parties are claiming this amount, the CRA will DENY BOTH. If the parents are not talking to each other, you (the tax preparer) may have to do the talking...
There's more to this answer: Perhaps consider switching the claim from year to year
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