Omfg! What an idiot! Whit that going on if I was you and my eldest showed even a slightest hint of understanding what's happening (which by the way I'm 100% sure he/she already does) I would say something immediately. You can just tell H that the eldest asked or seemed confused. I spent 3 months anxious every day about telling them (new me would have asked W once). It isnt that big of a deal. NOT telling them while they may understand or are confused, is worse IMO.
Me: 34 Stbxw: 30 D:5 D:3 Mini bd: May/June 2019 Married: Aug 2019 BD: 6th Dec 2019 OM Confirmed: Feb 2020 March 2020: I filed for D Waiting for D to be finalized and W to move out end of January 2021