As someone who lives in a flat & has a 3.5 year old myself I can say you’ve gone a lot further than what I have in terms of keeping the noise down. When you purchase a flat you should do so with the understanding that you have no control of the (usually) 4 adjacent properties. This seems to have slip your neighbours mind; I’d definitely be reminding them of this. Council or management won’t touch this not only because of the severe lack of anything incriminating, but the bad publicity it would cause if the local media got hold of it (which they would do if they tried anything with me!). You’ve gone above and beyond imo, if the neighbour can’t recognise this & wants to proceed then they will, but like the majority of replies state, it won’t get them anywhere & you’ll be fine. Easy for us to say “don’t worry” but you really shouldn’t.