My daughter is 5 years old and we currently live in the U.S.
A lot of Polish communities have "Polish schools" set up for 1/2 day on Saturdays. Catholic schools usually provide the classrooms. The program is way cheaper than what you would pay for babysitting and the kids learn the language and interact with other children. I'm pretty sure the program could be better, simpler, without history and religion intertwined with the general language, but it's an option you can look into. And definitely speak the desired language at home. My husband was born in USA and his Polish is AMAZING. All thanks to parents that cared and Saturday Polish school. Now we're raising another generation of Polish speaking Americans - our kids.