Depending on their age ...
Younger children; just let them be kids.
Even in the healthiest environments, kids are messy, forgetful, cheeky/rude, sometimes disrespectful, they push boundaries, get scared/anxious etc ... many emotions will very likely be stronger in children coming from unstable backgrounds, they will push you harder; but just remember they're only kids and most behaviours are learned. All kids love and trust unconditionally until suddenly they don't.
If you find yourself with one that don't want to eat their carrots ... ask yourself, have they ever even had carrots before? Have they ever sat up to a table to eat or have they always ate on the floor or sofa in front of the TV? Might sound odd, but it really can sometimes be the case.
Older kids (adolescents); all of the above, times a trillion!!
Kids that have been exposed to sexual abuse don't really understand what sexual abuse is, but an adolescent does.
A young child that's been hit will likely retreat immediately and then (when safe enough to do so) will literally bend over backwards to feel love again, an adolescent is fucking pissed off about it, about not being strong enough to fight back, or strong enough to hold back their tears. Their awareness is so so much more in depth.
Again, even in the healthiest of environments, teens are difficult, all those hormones teens have anyway, plus now you have social media ... boys feeling the need to be masculine, girls feeling unpretty or fat ... all that with a distorted view on what love & support is ... they're (I'd say) the most vulnerable.
I think it's amazing your even thinking about doing fostering 🫶