My ideas fwiw based on my kids, and also actually not being 100% familiar with heirloom present concept but think get gist.
Baby first Xmas mine were 6 months and zero idea what going on. Both were more interested in the packaging (box) rather than content. Like cats. The boxes were really exciting though!
So get fair few pics. Hopefully catch smiles. Sitting with family smiling holding out box.
Those pics when we look at it old pics kids like and grown ups all gooey.
Or can frame to get best and DC can see the love etc if first Xmas, so much pic is there.
I assume this is about giving things to mark occasion, and hopefully hand down if (if) they have children? To Sometimes look at and share feeling of love/ warmth, that sort of thing? Sorry if that's not right.
How about things that are lovely in general? That they will be interested in throughout rather than for little ones?
Eg
Good quality (made to last) locket or photoframe with pictures of them at first Xmas, with mum/dad?
Some kind of gorgeous little box thing made out of fancy wood well made etc. With card from you 1st Xmas, photos, anything. Letter from you about how you felt when they were born (positive obviously!). Maybe pics of any close family/ family very important to you, who sadly they will never meet?
That sort of thing.