Rule of thumb is up to twice the cal in grains of black powder is a healthy load. At 50 Cal. most will tell u FFg. 100 grains would be more than sufficient. You can load a little more and always less. You can also load FFFg, particularly in a shorter gun, but I'd never go over 100 grains with that. .50 cal. 50-grain "pellets" of blackpowder or powder substitute are commonly available (mostly used by hunters).
Probably uses #11 Percussion Caps. .490 lead balls and some thin lubed patches would probably be useful.
First, NEVER shoot anything but real actual true marked that way on the can BLACK powder- or a product made as a black powder substitute, such as Pyrodex. Yes, pellets are fine.
Second- NEVER attempt to fire a muzzleloader when the bullet is not all the way down the barrel on top of the powder charge. Having a gap between powder and ball will destroy rifle, and not make you look too pretty, either.
Read the manual, then come back to us with questions.
Call Mom, and ask them what there Baby takes. They may have the info you need on there web site. I bet if you called Hodgdon Powder Co. (1-913-362-9455) Tell them what you have they can tell you what you need. Don't guess! Bad things can happen. Go to YouTube, and watch a few how two's.