Pretty easy project, and unless the barrel taper is not a straight step taper, can be done easily with just off setting the rail off tram in a vise so you can use a ball end cutter for the needed mounting channel on fixed depth X axis passes.  Also, MOA can be added into the mount is desired as well.

As for if the barrel top surface is not a straight taper down at the mount point, then still can pull it off with a ball end mill, but will need to CNC the piece since your not just going to be making straight fixed depth passes down the X axis isntead.

So really, lets start with a photo of the barrel that you want the rail to mount to see if it a straight tapered at that section of not, as well as OD measurement at barrel at the start and stop of mount.

Not sure what your looking for on the Henry MiniBolt rifle, but there is the stock cantilever mount that installs without smithing.
https://www.natchezss.com/henry-22-mini-bolt-receiver-scope-mount.html

The downside for a youth and that scope mount, it has their head too far off the stock when looking down the scope for a decent check weld isntead.  Hence its the rear handle slot on the back of the receiver that the mount has to allow room for the handle to come back, so would pass all together on the scope in the first place on the rig; short of something with the scope pushed forward on the barrel to keep it low with a longer eye releif, and much lower on the rifle so the youth can have a decent check weld when looking through the scope.

Question, since you know that you need to leave room so the bolt has enough can rotate up and come back, it how far down the rabbit hole you really want to go.

Myself, would mill the cantilever block down to put it on the same plane as the front top of receiver, drill and tap the front receiver, then using the front of receiver top and block forward of that as secure points to bolt the pinnacle mount so you don't end up with flex problems.

If you already have the mount shorted to clear the bolt handle so it can cam up and back, then single point cantilever mount will work by just having the pinnacle mount surface milled down alone instead.  Combined with a smaller low profile pistol type red dot, this will drop the focal plan through the scope down far enough so she can get a good check stock weld.

Truth is, for what you doing, don't think that a red dot scope is really the answer, since without magnification, no better than a rear peep sight system isntead.  Hence on my lever guns since they need to hold to more abuse, have found that by just adding in a rear peep sight and reworking the front sight, can hold much tighter groups that you can with a red dot scope isntead.  Simply, with a red dot scope, the red dot covers too much area down range even with a smaller dot, so rear peep with thin front sight post is the better ticket to hold tighter groups isntead.

Going to make this simple, since the red dot scope is way too high over the bore line.

It not just that the lad can not get a good check weld to the stock to begin with, but with the scope mounted that high over bore line, the lad will have a hell of time trying to do elevation doping at even reduced yardages as well.

Hence the higher the scope line is over the bore, the more is has to look down to find the POI of the barrel, and this means that if you dial the scope in at say 25 yards, the bullet is not dropping fast enough to meet up with the scopes POA at that yardage, and rifle will be shooting high at 35 yards isntead.

So with a low profile type red dot scope, we can install that right to the top of lower profile mounting block, which keeps the POA of the scope closer to the bore line of the rifle, and this increases the distance that we can keep the POI of the barrel and POA of the scope on same point instead.  Also, this keeps the youths head glued to the check rest, so prevent parallax problems with their head bobbing around behind the scope as well.



So myself since you already have the catalever mounting system, would top mill that barrel mounting block down to  flush with the front of the receiver top (or make a new block), drill  the bolt for say a Vortex pistol scope mount so you can bolt that to the block (that is bolted to the barrel), have some of the red dot over handing the front of receiver surface to move it slightly rearward on the rifle.  From here, either lower or strip the iron sights off the barrel so the youth can get a good check weld  looking through the low profile red dot instead.