Is oral sex counted as a serious sin?
Yes, or at least it did 20 years ago when I was a Witness. It was considered pornea (is that the right word, I've tried really hard to forget that nonsense), and it was equated to a homosexual act. Of course since gays kiss, and kissing is okay, I never understood the logic of that connection. Either way you had to grovel or be DF'd.
maybe in because abusing children is a worse sin they need more grounds for evidence
No. The "logic" is based around the Mosaic Law with regards to settling a dispute (a dispute!) and something Paul said about needing "two witnesses to establish a matter".
So child abuse is equated with a dispute, whereby the child is automatically assumed to be lying (and we know how Jehovah feels about liars). The only way for the child to be innocent of lying is for the offender to confess OR for there to have been 2 eyewitnesses, preferably 2 Jehovah's Witnesses in good standing. These 2 eyewitnesses must stand and watch the child being raped without interference, because if they interfered there would be no child abuse now would there, and then they must wait for Jehovah's spirit to move the elders to contact them because if they reported the matter to the elders that would (1) be running ahead of Jehovah; and (2) potentially bring reproach on Jehovah's name because the child may want to keep the matter private (in other words be shamed into silence).
This was what the circuit overseer in Texas District No. 13 told me in 1989 and was backed up by the district overseer. I used to have a printout of the Flock book bookmarked to where specific instructions are given to elders about procedure regarding child abuse. If memory serves, the official procedure is not quite so arduous, BUT the elders are permitted to ignore testimony given by worldly people and/or children as they see fit irregardless of what they saw and testified to the elders.
In my case, I WAS able to produce 2 eyewitnesses but they were rejected because (1) they witnessed separate rapes; and (2) they were worldly and we all know how worldly people lie.
As far as I know one of my rapists is still a ministerial servant in good standing. The other 3 are dead.
Plus most people accused of oral sex generally admit to it?
I never did. I saw nothing wrong with it then, and I don't now. And I damn sure wasn't going to tell some nosy horny old goat of an elder what does or doesn't happen in my bedroom!