What the fuck man.
GO SLOW.
You're introducing her to your fetish. You're lucky enough she's willing to give it a try, she's most likely into the "cute factor", don't force her into your fantasies, at least not just yet.
Honestly, I'd start by giving her a nice hot bath, maybe with baby shampoo for the immersion effect, then diaper her slowly and carefully, making sure to rub baby oil and/or lotion and/or powder into her "intimate area", as slowly and sensually as you can.
Make her love it, appreciate it, maybe even get off on it before you go forcing her into anything, potentially risking to scar her and ultimately ruin your relationship.
Once she's in a diaper, unless she asks to be dominated/treated like a complete baby, I'd let her take the lead, be the nice daddy who just wants to spoil his daughter, see where she wants to go with it, unless she has no idea and clearly wants you to take the lead.
Either way, don't lock her in diapers or force her to wet/mess, let her take that decision, or at the very least, make her think it was her decision. A pacifier and baby bottle probably isn't too hardcore, girls in raves used them all the time back when I was in that scene…
Once you've given her enough to drink to ensure she'll need to wet soon enough (make it alcohol to lower her inhibitions and make it harder for her to hold it, if you really want to "force" her into wetting herself…), if she asks for the bathroom, tease her about the fact she's wearing it, and gauge your next move based on her corporal language, if she looks disgusted at the suggestion, don't insist, let her use the potty and hope she'll open to the idea.
Anyhow, it's your GF, you know her, I don't, so maybe your plan will work, but honestly, if you're not 100% sure, I'd go as slow as possible and make sure you have a safeword so if you do anything she doesn't want, she can get out of it.
Then again, if you've done BDSM or other kinky stuff with her in the past, maybe my suggestion is actually too slow, especially if this is a one-time thing, in which case you might not be able to make the most of it, but really, unless you're 100% sure she's on board with being "forced", don't force her, use this first experience to initiate her, not to test her limits.
On the other hand, you can make it easier, maybe, by being her playmate for the afternoon, instead of her daddy. If you're doing everything you want her to do, she might feel less humiliated by it and be more open to try things she wouldn't want to do on her own.
Anyhow, good luck and keep us posted, I need to know how this will go.