Hi, we have a birth daughter so had experience with her but were told to get experience with other children too.
I was part of NCT already, and so I was lucky to get involved in things as a parent. I also got involved at the local pre school.
Places to try would be:
Baby groups, toddler groups, Pre schools, nurseries, creche facilities (at churches or other places of worship, sports centres etc).
You will need to approach organizers of 0-3 activities and I'd suggest you explain why you want to volunteer. Whether the organizers pass on why you are there could be up to you. EG they could say you want to work with kids, if you wanted to keep the adoption private. But I would be completely honest with the actual organizers.
Things you could do to help could include reading stories, organizing craft activities, helping with snack etc.
Read up on child safety, things like chocking hazards - the right way to cut a grape, it could all be useful.
Good luck.