The kick of a BP gun is relative to the amount of powder you shoot and the weight of the bullet. I plan on using an inline with about 60 grains of powder and a 240 grain bullet for my son's first deer gun. Personally, I shoot a BP under 7lbs with 80 grains of T7 and 300 grain XTPs with great success and not much kick. XTPs perform great with light loads since they're a pistol bullet and no issues with performance out to 100 yards.
if you go with a 410. Rossi makes a 410 that is very light and has 2 barrells where he can shoot the rifle and change the barrell in a matter of seconds to shoot the shotgun. you can get one for bout 179.00 bucks. i have a ten year old and he killed his first deer with it at 5 yrs old. he now shoots a rem 870 youth model pump. has killed a tractor trailer load full of deer. hope you find something.
I started out with an old Springfield single .410 it didn't have noticable recoil, it was great for phesant & quail, cheap & safe.
I gave it to my son for his sons to start with. As for deer hunting most slug guns are brutal for a youngster unless they have recoil reducers.
I'd be looking at a 243 if you hunt deer in rifle countys. Who knows maybe the Rossi might come in both???
Forrest Seaswirl striper 2300 wa
McKee 17' Marathon cc
Both of my boys started at 10 with Mossberg 20 ga. youths, and they've worked out very well. The gun is a bit more than some mentioned (I think it currently goes for $360 or so) but it comes with both deer (rifled) and waterfowl barrels, so it covers all the bases. When the boys go to 12 ga's for waterfowl, the Mossbergs will still make good slug guns. A friend of mine has a son who's 18 now, and still uses his for that purpose. Allaround, we've been very happy we chose them.