Your daughter cannot renounce her Canadian citizenship under the age of 18 years. It would have been a different matter if she had applied for an Indian passport first.
You can apply for an Indian passport for him IF you and/or your wife are Indian citizens.
That said, because he was born in Canada, he may not be able to get a TRV/visitor visa to travel back to Canada if he leaves the country. His immigration status will always show as a Canadian citizen.
Answered above
Another solution for you:
1. Allow your children to enjoy the privileges that come with having Canadian citizenship. They don't have to 'work' to get it.
2. Apply for OCI cards instead.
3. Have you checked school fees for really good quality schools in India? You might be surprised that the cost of schooling in prestigious, good quality schools is the same as abroad. Of course, public or government schools have negligible school tuition fees. But then you would not want to send your children to public or government-managed schools in India, right?