I have several pictures of myself and DD1 on one my walls, one taken on the day we finalised, in our nice clothes, and one in introductions
I've picked DGD1 (she's nearly 2) up a couple of times so she can touch the faces and said "look, that's mummy when she was younger! With me, grandma, when mummy came to live with me forever", and "look, that's when I adopted mummy" etc.
That's it for now, just little "drips" about being adopted and forever families, the two girls can pick up on. At some point relatively soon, I'm sure the subject of pregnant ladies will come up or something DD1 can use to broach the idea that grandma didn't give birth to mum. Or maybe GD1 will ask what adopted means.
Either way, no one wants it to be a secret, or a big reveal when they're teenagers. Of course it's DD1 who decides and will continue to decide what she's happy with me saying to the little ones, but we feel the same way about it all which is good. We both feel that if they got to teenagerhood before finding out, they might have formed or internalised a negative opinion of adoption without realising how close it is to them, so it's better that it's just one more fact about the family they always happened to know.