Here's the cool thing about this. Once your kids cross into the US with immigration visas and are under age 18, they automatically become US citizens. This means that they will not need the I-864. This means your Joint Sponsor only has to sponsor your wife and not your kids. This means that a single person making $21,000 could sponsor your wife.
If you want to meet the I-864 for your wife alone, then your household count would be 7 - you, wife, and kids. You would need to make approximately $50k. This means that if you have $150k USD in assets that you could cash out within a year without causing a hardship, you could sponsor your wife by yourself.
