k (if kids eligible for 500+ scheme)
If your family is eligible, you cannot collect for the oldest child. So 500 PLN max, if that.
2500 PLN is not sufficient for decent accommodation including bills?
Probably not. You're going to have to make sure that your apartment is conveniently located near transport to your workplace and your children's school. Cheaper apartments tend to be inconveniently located, or have other problems. Budget 3200 PLN for a basic, no frills two-bedroom apartment.
Oh, brother! You really underestimated this one. A lot. Unless you have had your wife and kids taxidermized, they are going to cost you a lot more than this.
You have a wife and two kids that will be in a strange foreign country where they don't speak the local language and will require abundant recreational activities to keep their minds occupied, especially during the long, dark and depressing winter (twenty-four hours a day, every single day, for five long months). You're going to have a film-worthy case of cabin fever before Christmas comes along.
Your wife especially is going to need some recreational activity that forces her to get out of the apartment and encounter other human beings or she will turn into a bitter, anti-social hermit or sink into depression, or just demand to be sent with the kids back to India before long. And I don't mean shopping and doing chores for the family. That's going to cost money.
The kids will also need some activities like sports, music lessons, language lessons, or the like or they will deteriorate into couch potatoes.
Really, with your ambitious savings goals, it would be far, far easier on everyone if you were to leave the family at home in India.
And even more so, if saving money and/or paying off debts is a priority, then moving to a country where savings potential is abysmally low like Poland is an unwise move, especially considering that the cost of living for you as a foreigner with school-age children is very high. Pick a country where your savings potential is much higher, the quality of life for your family would be much higher, and the cost of keeping your family is not as oppressive. Like one of the English-speaking countries, where you and your family will be able to communicate with the locals.