my 2.5, 4 and 6 when we started our assessment, we included the 3 of them all the way through the process, and told them what fostering was, what confidentiality was, explained that they were our "forever" children and that they would never leave, but the children that came to stay would never stay here forever, but would go either back home or to new parents and they would then become their "forever" children. We did reinforce the the children will always leave bit a lot!
We were very open and honest about fostering to both the children and the social workers, the social workers did want to see how we functioned as a family, and they did ask if we would mind them speaking to the children when we were not present - so that the children felt under no pressure to say what "we" wanted them too. The social workers were most taken aback when we said no - do you want to do that just now? which is exactly what happened, this was closer to the end of the assessment so the children were very relaxed and comfortable with the social workers.
I feel that all 3 of the children had a good understanding of what we were undertaking, as it turned out due to personal reasons we took time out from the assessment process and our littlest was just under 4 when we started, I do feel that the social workers included him in chatting to the children more to make him feel involved rather than getting much information from him, and equally I do feel that they had a chat with the children rather than questioned them.
I feel that all the ground work and chatting with the children has really paid off, we are now over a year in and the children love being part of a family that foster, they just adore having an extra playmate in the house, and there have not been any issues at all in regards to how the children have dealt with the FC's or jealousy of the time my husband and I have to spend with the LO or having go to meetings.