You lost me when you said you moved schools for on with better status
Me too. You will need to have explained this situation to your children and it sounds as though they didn’t want to change schools. Why would they?! They had already had enough upheavals. It sounds likely your youngest started playing up out of protest.
I also don’t think you know your children well enough to make a judgment as to whether or not they are lying. Some children (and indeed adults) readily lie. Others do not. If your children thought you were going to punish them for bad behaviour, of course they’ll lie. I was pretty truthful as a child used to deny something if I thought I’d be punished. The trick is to tell your dc, you won’t punish them for doing whatever naughty thing if you just tell you the truth.
Somewhere along the line, your children have received the message that your families status is higher than that of the hired help (and yes, I’m being deliberately derogatory right now calling her ‘hired help’). This is hardly surprising really, when you’ve systematically undermined her, believing them over her. And you cemented it by sending them to a high status school, nanny didn’t attend one of those, did she?
I also think you’ll go through plenty of nannies if you don’t start listening to them. And start thinking more about your children as people, who need to be guided, respect adults and given the opportunity to be truthful without judgment.
You are bringing up future adults and allowing them to lie and believe they are superior to others of ‘lower status’. This really is not going to end well. Potential narcissists in the making.
I do have experience of the private schooling sector btw. It is far more important that your children thrive than go to the highest status school.