As worrying as this situation is you need to nip it in the bud and correct it right away, they are both old enough to understand that they shouldn’t be touching each others private parts ete or doing whatever they are doing if you have already told them to stop doing it ete
You need to be concerned that something is wrong with them or someone is touching them.
Many children since the beginning of time have caught their parents and or seen them naked… the normal more prominent response is total discuss and removal of eyes… not ohhh that looks fun … let’s try it out brother.
Honestly at that age they should be a little confused about what they saw.
You need to teach then modesty and appropriate relationships… if they will be inappropriate with each other they will abuse other children as well.
And you don’t want other parents or the school calling you. They will assume abuse which I am too.
My parents fostered children and this was common behavior for those that were abused. Some were traumatized and some abused other children.
Do what you have to.