Advice at telling my children I’m adopted

Hi, So I’m now 43, I found out my dad isn’t my real dad when I was 12, he took me for a walk and told me that my real dad died when I was a baby and he married my mum and adopted me, this is literally all I was told. My brother is my half brother and I don’t know any family on my birth dads side. The difficult bit is we are not allowed to talk about it, my mum will not discuss it at all, and non of her friends or family are allowed to talk to me about it, I don’t even know if my brother knows! I have read the coroners report now, so I know my dad committed suicide because of financial difficulties but I got this info on my own. The problem I have now is my children are 9 &10. I don’t want to lie and keep this from them but How do I tell them grandpa is not their natural grandpa? How do I tell them the truth and then say ‘but you can’t talk to grandma and grandpa about it!’ Any advice would be greatly appreciated.