How do I open and permanently disable the camera?

In terms of anything physical, none of them work well on Macs these days as they're not a way to open the screen and the webcam cable is integrated into the display cable on Macs, it's not split. The older ones could be done but they require a heat gun and it's hard to reverse. I wouldn't recommend tamper-evident tape or a cover on the display directly either since it may damage the AG coating and covers tend to break it these days. Your best bet is to see if there's 3rd party software that can block it, or if you can use Screentime.

Your needs may be better served with a used PC, like an older business notebook. On many of these, the cam can be left installed with a BIOS block (disabled in BIOS and doesn’t show in Windows) or you can physically disconnect it since most PCs are more friendly to such a thing. It’s the nightmares like my FHD Touch E7440 you need to watch for - it’s built in such a way I can’t take it apart without ruining a $120+ USED assembly easily. If you do not want to deal with it being present at all, you can sometimes choose lower spec models like my E6440 pre-display assembly swap (i5-4300M/4GB RAM/768p TN LVDS LCD) and they usually do not come with it, or look for a surplus unit that lacks a camera; though take note it's probably government or business surplus and was treated as such. Most manufacturers dropped the webcam delete option for consumers years ago. Covid note: A lot of these are more ready than sometimes nicer “high-risk” machines. It's not easy to find one that’s ready to use due to higher demand form parents.

Example of what to look for:

AVOID (IPS; panel looks really good, but the design makes camera removal difficult):

GOOD (poorer quality LCD, but easy to remove without damage):

However, be careful about displays - when I got mine, the display was bad (I got it cheap because of that). In my case, I found a HD+ (900p) webcam assembly was an extra $4 because of cosmetic damage I can live with for a better panel without having to muck with the cable. Yes, the E6440 LVDS has different cables, while the iDP has one (iDP has better panel upgrade viability). However, it is a horrible machine if she needs to carry it (thicc+~4.5lbs).

CAUTION: On the business machines, you need to be a little careful about Computrace being active. If it is, check with Absolute SW and provide your proof of purchase and they’ll help you remove the tracking from the machine, but nobody can change the persistence module - not even Computrace. But it can be “neutered” with a POP and an eMail.

In terms of the cut: Don't cut it - it makes restoration hard for you to reverse, or if someone buys it secondhand hard for them. Disconnection or removal is usually sufficient, especially for someone who isn’t there technically. Cutting it devalues the hardware to tinkerers. I personally hate machines someone ripped the cam out AND cut the cable. If I'm dumping the assembly (or at least the back lid), I’m still annoyed but I consider it neutral because it never mattered. The issue is if it's in nice shape and I do not need to replace the back lid, I'm looking at a 1-2 hour repair to reverse someone else's heavy-handed parenting. If possible, disconnect the module but leave it in the notebook so it can be quickly reversed so you can also go back when she's older.