It happened to us, I even have a thread on here about it. We were told by accident, they thought we already knew. We were told a few months before the end of the school year - the kids were 5 & 6. I have a belief that we should do everything we can to ensure that by the time our kids are older there are no big shocks or surprises if we can help it. We do our best to gradually introduce things as they grow age appropriately.
We waited until the summer holidays and then told them. Before school finished though I informed the school about the news so they were ready for anything when the new school year started.
My kids had experienced the deaths of pets, and we spent the few months before the holidays talking about invisible strings, going through life story books subtly and the like.
The children cried, they didn’t remember the parent, but they knew the news was significant. We told them we could plant a tree in our garden.
We were right to warn the school, my eldest started to get into trouble a bit at school, he was easily led and couldn’t regulate. But school swept in and literally through a safety net around him. They gave him a quiet area, provided ELSA support, and made sure all his teachers knew what was going on. It was only for a few months, but it did feel like if school hadn’t have supported him it would have continued and escalated. Keep it in mind though that all this came after Covid and lots of changes at school, so I don’t think the behaviour changes at school were all due to the death of the birth parent.
Then six months later they lost a beloved adoptive grand parent - that was pretty awful, but it was awful for us all. My eldest just threw up when he heard the news. But I think it actually helped them to see us upset too - we talked, we remembered, eventually we laughed about funny things their grand parent had done.
They are both doing really well now, we talk about things a lot. Death is a natural part if life, it’s hard, because we want to protect our kids that have already been through so much, but I think they do need to know. I’d just say pick your timing to the best of your ability, and get your support systems in place. We are not quite sure why our birth parent died…neither was the coroner really, so we told our children that we didn’t really know, only that we knew they weren’t very well mentally (sorry clumsy wording).