E.S.
I think there has to be more to the story. If what you mentioned happened in my family, I wouldn't think anything of it. But my dad never did anything even remotely inappropriate to us growing up so I certainly wouldn't ever make a jump from helping my kindergartner wipe to something sexually deviant. My 5 year old sometimes whines and asks me to wipe her (I politely decline!). You said you reminded your daughter to wipe and that's how this whole thing came about so she isn't past the stage where help is completely inappropriate. Honestly, making that jump seems so extreme to me that I would label it something well past being overprotective. It makes me think there is something else out there.
Did something happen to you or someone you know? You may be projecting those feelings and becoming over-zealous about the issue. If something sexual happened to you from your father, I would think you would never have dropped them off in the first place. Think back to how your father was around you and why you don't trust him and talk to your mother about that. I wouldn't tie it to these incidents because they sound completely harmless to me and I'm sure they would to your mother also just like you thought. At the same time you can't discount your feelings about why you don't trust him, I really can't imagine it's because of these two incidents, so you have to figure out why it is you don't trust him. And while you are trying to figure it out, get another sitter!
Hope this helps!