I don't know about DSs, but DD2 could be very negative and moany, especially as she walked out of school.
In the end we had a rule, say something posative or just shut up.
She was very well behaved at school and worked very hard at fitting in and being a good friend.
As soon as she got out she had a massive moan about absolutely nothing.
I think, Y5 and Y6 are stressful, DCs aren't grown up enough for real freedom, but they are leaving toys and childish things behind.
They feel really frustrated. Theywant more control over their lives than they can have or actually handle.
Sometimes this frustration comes out in clear toddler grade stroppyness, sometimes in teen attitude and sometimes just in low level grumpy moaning.
They focus their frustrations not on big things, they know they can't control those, but stupid little things they feel they can. Trouble is their choice of things to get totally annoyed about is, to everyone else, not always logical and very wearing.
The best response I found was giving them the opportunity to feel grown up.
Independence and being able to do things with their friends and a say in family days out etc.
Really little things like KFC rather than Macdonalds. A hour in town with a friend on their own or going swimming. Staying home while I taxi her sister about or getting to bake.
I don't do screen limits as SIMs is DD2's escape, but she has never kicked off hugely about being told to do something else. Like many if your DSs she also needs to be sent out to race about or trampoline.
Also I'm certain my DDs sence that I'm far too lazy to enforce ridgid time limits and certain to forget to look at the clock. In any case I feel petty rules for the sake of rules live in school.
I don't worry to much about screens, bedtime, chores and general untidyness.
I'm am strict about attitude to parents or siblings, working at school (not every scrap of HW, but overall attitude) and looking after possessions.
I also dole out lots of hugs.