Hi, I'm a 34 year old woman with a cavernous venous malformation on my neck and jaw. From my point if view of being a young child growing up with the condition, and now being a mum of a 2 year old with a PWS, I would advise to wait for the questions, and when they come answer them truthfully, and give as much detail as required. I think children can cope with the truth. I would also keep your approach quite lighthearted and casual.
My parents always talked about my VM openly but I feel they shielded me from the medical knowledge behind the condition.. Or maybe they just didn't know themselves. I had 'extra veins'. I did all my own research when I grew up. I feel that a parent should know as much as they can from a medical perspective, so they can answer questions, because as the child grows older the VM may change and the child may want to know what's going on and why and what to expect . Also I think a realistic approach to potential treatments is important ., no false hopes and sweeping statements like 'this op will cure you' because kids believe everything you tell them and it mightn't go to plan. So realistic expectations I would definitely advise. But confidence wise, children are amazing at just accepting what's there.. Parents can be the ones more affected! Hope this helps and good luck.