Thanks for your advices!

You are maybe right, I'll try to buy something like @hebe posts, small front seat, anyway, my child is't big enough to put it in a trailer. But what to buy then, that's a question... In fact, there are not many cycle paths in place where I live, and this is why I'm a bit scared about using a trailer in a traffic. You don't know what is happening behind you all the time.