What helped with my DC-
Routines. Bedtime, bath time, going to school (applicable for my DD and DS, maybe not your fc), getting up, birthday routines or traditions maybe.
Food. The foster carer of DD1 and DS wrote a basic food diary of the week (very simple) so I got an idea of the type of meals (and how picky they were!) and their likes and dislikes. Is there anything specific which appears to be important to your fc? Even if it's like supermarket own brand or not, or if there's a certain food they like to have when they're ill, or comfort food type things.
Shampoo, washing powder etc; brands for familiar smells/feel.
I bought DD1's duvet cover from their foster carer as apparently DD1 was very attached to it (hated when it was being washed type thing) and paid for a replacement. Is there a version of that for the dc?
Any vague interests. DS, for example, loved Chelsea FC, so we looked up a few players and when we found out he liked these sport card things, it meant that when we visited him, we could ask to see his collection and discuss them a bit, so we had a bit of common ground and it meant that instead of him focussing on us being new people, we could be bonding over something.
We also got told of their favourite games, their favourite ways of trying to get out of trouble (like any stories), the imaginary friend of DD1 (who she blamed things on), any information they might want to know about themselves when they were younger (funny stories etc;).
It's been six years since then, and the foster carer isn't a foster carer any more, but we are still in contact. We get birthday cards and presents, and we email them photos and so on. DD2 (a separate adoption) unfortunately the contact isn't so great, mainly because we didn't establish the sort of contact we wanted I think, although we still have some contact and have put in effort.