There's your problem, you are posing it as a choice
"Do you want to do a nappy?"
No, of course she doesn't if theres another option.
Tomorrow, be matter of fact. Give direction and positive enforcement and lots of chivvying on as she does it and narrate her way through.
" Ah, it looks like Sarah needs her nappy changed. Right, I've got her, you go put the gloves and apron on and you can change her. It's alright I'll be right next to you. Right, thats it chat to her and make eye contact... Great yep, it's probably easiest for you to take her tights completely off and pop them back on after. Great, yes they can be wriggly you are doing a great job......."
Just because she's been shown doesn't mean she has the confidence yo do it, you might find lots of chivvying and praise to get the first one done will work wonders and then future ones will be far easier.
Your choice of language is so important. The best advice I was ever given by my mentor is "don't say please", 'please is a choice',
"Please can you put the chairs on the table" - there's wiggle room if they don't want to
"Put the chairs on the table, thank you" - no wriggle room, you expect the instructions to be followed as you've already thanked them.