personally, i wouldn't use old ink buckets around kids. I am about the furthest thing from a granola-eating-all-natural-organic-type person, but knowing where they came from and what was in them, I would buy new in a heartbeat.
check with your ink supplier, they probably will sell them cheaper than paying a guy to clean them out...but this post will probably be ignored.
CLEANING OLD BUCKETS ISN'T WORTH THE HASSLE.