at Polish preschool in September
On the basis of personal experience, I don't think English preschool in Poland is not a good idea. By the time you get to Poland she will have a good grasp of English which all you need to do is keep it up by speaking at home. In a Polish preschool within 2 months she will be chattering away in Polish like a antive, so outsiders wouldn't be able to tell that she isn't born and bred Polish. Kids naturally take to their peers and through conversation, games and general interaction sop up a new language like a sponge. Good luck!
is not a good idea
Please delete the superfluous "not".
The sentence should read: I don't think English preschool in Poland is a good idea.