I live in a downstairs flat, with one flat above me. The owners have two young children (a 3/4 yo and 2yo). I absolutely get that if you live in a downstairs flat you get noise and within reason I will tolerate it (and moving out is not an option that's going to be available for some time). I also get that young children make noise, so within reason I'm willing to be even more forgiving than I would otherwise be.
The issue seems to be (I don't know because I've never gone inside) that they've either removed their carpet, or replaced it with something more flimsy. And they, and their kids really do like to dash about - from the sounds I hear, I think that one of the kids likes running, in circles, in the sitting room, (which is directly above ours) and in the kids bedroom (which is also directly above ours). They also do something which sounds like jumping from their staircase onto the floor below. My ceiling rattles with this.
If it were for a short period or a few times a day I could cope, but it is starts about six in the morning and will continue for between 10 minutes to half an hour every hour or so. When it was getting dark earlier I think the kids went to bed earlier because it stopped around 630 but at the moment its going onto around 830.
I can hear it over music and over the television. It is a really wearying and grating sound - nothing like regular footprints (which I also hear and can put up with more easily) because its got an urgency and a repitition to it. On days when I have to work from home (no choice sadly, and because of company equipment no I can't leave the house to do it either) my neves are absolutely shot by six or seven.
So would I be unreasonable if I politely approached the parents and asked them if they would take some steps to modify the noise - preferably carpeting or putting a big rug with something under it down in the two rooms affected or by trying to get their kids to play in some other way, at least some of the time (they have a garden and there is a park opposite us, BTW). I would happily contribute towards the cost of carpeting.
I could, technically, speak to the freeholder about this, but I'm reluctant to do that because a) I'm worried that it will blow it up out of proportion and b) they might respond that we don't have carpet in one room (our sitting room) - which is true, but not strictly required by the lease and as we are the downstairs flat the noise reverberation is less of an issue.