It depends to be honest
Most adoptions are very young children, people dont want to adopt older children so offers start tailing off for 5 and upwards children, although we do still have those ages adopted
For baby adoptions the child may either never have been in parents care or have been in their care in a parent and child facility where their care has been supervised and monitored
For slightly older children, 18mths/2/3ish, it depends when they have been removed. They may well have been removed on a single issue incident, or they might have been removed after 18mths or 2 years of utter neglect/DV/abuse
They might have been the child that had the good care but proceedings commenced with older children, with the conclusion that the younger child will soon be exposed to the same poor care as they get older and so the prognosis of significant harm is there and the LA is issued with a care and placement order for adoption.
But, whether or not there has already been significant damage via poor parenting, the child will always have suffered loss, disruption, instability by the very nature of the process, its unavoidable and that damage needs to be supported by the parenting they receive while in foster care but also later with adoptive parents.
Which is why, even from a baby upwards, their worlds must be very very small and still after transition and placement and big 'do's' with them included are not advisable.