They are teaching their grandmother to suck eggs
= to give advice to someone about a subject that they already know more about than you
Cambridge
Perhaps its meaning is getting lost in time as few people nowadays literally suck eggs. Many years ago people would suck out the egg contents by piercing the egg at both ends and then sucking on one of the ends. You could reverse the procedure and blow out the contents also. It was such a commonplace procedure then that to "teach your grandmother to suck eggs" was like a child trying to teach as new something the grandmother well knew how to do. The saying still survives despite the fine art dying out in our "civilized" and salmonella fearing culture.
English Stack Exchange
You may also like the relevance of "Queen Anne is dead", an idiom discussed well in:
the death of Queen Anne was officially hushed up for a while [...] News had leaked out, so when at last there was an official announcement of the Queen's death, the crowd chanted in derision "Queen Anne is dead - didn't you know?" and to this day "And Queen Anne is dead" is a standard rejoinder to somebody who bears stale news or states the obvious.
English Stack Exchange