This issue has come up several times since lockdown began and I daresay it has been aired many times before. it is a perennial problem, lockdown or not.
I have not entered into the discussion before now because the majority view seems to have been that noisy children are all part of 'urban noise' and those who are troubled by it should get over it. However, I notice a change of majority view in this thread.
We live on what would be regarded as a mature estate. There are houses with young children, but the majority are empty nesters (like us) or have older children. The house that backs on to our NDNs' garden has noisy children and we and the NDNs have had to suffer every summer since the family moved in about 5 years ago. The children will play happily for so long until one will antagonise one or both of the other two and then the screaming starts. One of the children continually calls for Mummy and it can continue for some time until a parent intervenes. As a parent, it can be very stressful to have to hear. A couple of summers ago NDN raised the issue of the noise with the parents over the garden fence and they agreed that their children were noisy, but seemed clueless as to what to do about it.
So, fast forward to this summer/spring. A few weeks ago NDN discovered these children in his garden and he completely lost it. We heard him yell to the parents who were indoors, blissfully unaware of what their children were up to. The mother eventually came out and we heard him say that this was the last straw, she and her husband had no control over their children etc, etc, etc. We actually found the whole thing quite troubling because we have lived next door to these NDN for 32 years and an outburst like that is completely out of character.
So we wrote the parents a letter, explaining that we were shocked by the outburst, but that NDN were not the only people who were disturbed by their noisy children and asking them to exercise a little more parental supervision so that we could all enjoy our gardens in the way we want to. We spent ages trying to word the letter diplomatically, in non-accusatory terms and appealing to their sense of reasonableness. Lockdown meant of course that we couldn't go to their house and speak to them, so writing a letter enabled us to choose our words carefully. The letter was duly popped through their letter box.
A few days later, we received a written reply from the parents. It was apologetic and they promised to try harder, although they could make no guarantees as their children are young. Fair enough. We wanted to rush round to their house and thank them there and then for their helpful and conciliatory response, but again lockdown prevented us. NDN did however thank them over the garden fence for their helpful attitude and hopefully any ill-feeling has been resolved.
A few weeks on , we now hear almost no sound of the children from their garden and in truth we feel a little sorry for them. We didn't ask for silence, we just asked for the parents to be a little more vigilant and to intervene sooner when either the children call out for their parents or WW3 sounds imminent.
The OP sounds as though she exercises the right amount of supervision and is well aware of how noisy her children are. As the parent of four grown-up sons, I would say continue to remind your children of the need to be aware of other people and how they might feel about noise in the garden. It is a long and tedious process, but your children will be all the more considerate adults for it.