Medical cost should be enquired with the hospital directly. I don't think anyone can help here. But for staying in Bangalore Rs 2000 per day is enough for a family of 3. There is an app called OYO which have standard rates. You can check exact prices there:
Hotel Booking India, Branded hotels, Affordable Stays – OYO Rooms
In a normal corporate hospital, a decent bed cost per day will come to around Rs 2500 if the room is shared. Food might cost 500 more. I am not sure about kids, they have special needs.
If you are planning for 15 days stay, I would suggest you keep 1.5 lakhs cash for expenses. This is of course apart from treatment cost which includes surgery and medicines.
Paying guests will have problem hosting foreigners I think.
@BDforever I mentioned prices for a comfortable stay. If they can manage much cheaper places, they can save on it.