When they remove child porn images, they tend to leave the original posts and their accompanied notes intact. So, basically, they're leaving up lots of these "Hey, we removed illegal material that was here, but please click below to see 200 more blogs that might have what you're interested in". They are essentially accomplices to distribution of child porn.
The child porn pictures often get reblogged over a thousand times before they are removed, which can take a week or even months. There are a lot of child porn images that have been on the platform for several months, even though numerous users (but not all of them!) reblogging the illegal image have been banned. Even worse, banned users just recreate their accounts, sometimes incrementing a number in end of the account name. Those numbers eventually reach double digits, they just keep coming back.
There are also "beacon" images that don't get removed at all, since they only contain text in the image such as "reblog if you want to have sex with children". Using these, pedophiles can easily find hundreds of other pedophile blogs and reconnect back to the grid after they are forced to recreate their account. Some of the beacons have existed for several years.
The pedophiles on tumblr are fearless. There are people taking selfies of themselves, showing their face, and then showing them jacking off to child porn that's on their computer screen. There are people openly trading their phone numbers to connect in Whatsapp to trade images there, and although the international numbers I've seen have mostly been China, Vietnam, Mexico, Brazil, and other similar places a bit further away from the western civilization, there are users from Europe and USA posting their phonenumbers on those beacon posts. There are posts about "pedo pride" and the whole thing is outright crazy. There are thousands of active pedophiles on tumblr. THOUSANDS.