The 45-70 can be loaded up and loaded down. If you check an old Ideal cateloague, they included loading instuctions for very light gallery loads for 45-70's, and even for small game hunting. Back in the day, some folks even loaded 457 round balls in their 45-70's. I have a mold for a 210 grain 45-70 wad cutter style bullet that when loaded with some felt wads and seated all the way into the case, packs about as much kick as a 38 Spec.
For the 44-40, when my daughter was 11, she loved to shoot (her pre boys and driving days) We loaded up about 300 44-40's with lead bullets and some really slow burning rifle powder (4198 maybe?) It had the kick of a 22 mag. Very pleasant to shoot. For plinking in fact, it was a little too enjoyable. Needed to reload again too soon. For a real learning experience, I have an old single shot rifle in 357. I let my daughter load black powder loads with a lyman nut cracker tool for 38 Specials. She was shooting great offhand groups out to 100 yards with it. (Wiping between shots.)
I would avoid Pyrodex. It is more corrosive than black powder and just as hard to scrub out. May as well use the real stuff.