It has been my experience that a .410 as a slug shooter is far more accurate than most folks give them credit for. I believe they are certainly adequate for deer if one is patient and keeps their shots under 50 yds or so. Preferably under 65 yds at the worst case scenario. It would be suggestion that one get an H&R youth gun. (Sgl shot) I don't think they come in .410 with rifle sights, but could be fitted by a competent 'smith for a reasonable price ......
H&R / N.E.F. (New England Firearms) makes a .45Colt / .410 SG "Survivor" that has a 20inch barrel that's rifled, but for the last couple inches of the muzzle - where a choke is usually used when shooting the .410 to stop it from spinning, but removed when using a .45C. I have no idea how well that barrel would work with .410 slugs, but check it out.
www.hr1871.comBTW, are you already invested in a .410? If you're still makingf your decision as to what to start your youngster with, NEF also offers a fully rifled youth-model or adult Tracker II barrel in 20 gauge (and 12ga), along with their 12 and 20 gauge Ultra Slug Hunters.
If I could talk you into using a 20ga, I'd suggest giving the 20 Tracker II a close look, plus its not that expensive!
But Good luck whatever you do.
BTW, I have NEF single-shots and a Remington M870 Youth Combo (which I wanted for its fully-rifled 20in bbl and its 21in RemChoke bbl). My single-shots don't include a .410, altho I would someday like to pick up a .45C/.410 bbl for my T/C Encore.
If you go the NEF "route," most folks I've heard report their results like Forster style slugs for their acurracy over other slug designs. "Forsters" are usually the cheapest!