I think there are some reasonable copies made at Riceville. They are great, just be sure to connect the back attaching strap to the girth D ring to keep the back of the saddle down on the horse. The 5 y/o can gradually move into no stirrups on the buddy seat, this will greatly improve his balance.
Kids aren't generally ready to "ride" alone until around 8. Through using a buddy saddle my kidlet started being ponied by the time she was close to 7, on a good horse, then by age 8 - 1/2 we were able to let her ride "solo" ie not being ponied, on a GOOD pony.
I teach my horses to accept a floppy legged buddy riding behind the saddle (or as the rider when first preparing for a rider) by using one of those long cloth tube things you put deisel on so cows go under it and get it on them for fly protection. I let mine get wet, so it's heavy and floppy. Works great! Nobody gets hurt. The ones at Henyard have strips tied around them. Great desensitization!!
PS I never rode with one in FRONT of me...i never felt safe doing that I was afraid I'd squish some important body part (theirs or mine). I started buddying my kidlet when she was 4. She would ride behind me all day long singing and swinging her feet then gradually she went to the "airplane" ride with her arms stretched all the way out flapping, yelling WHEEEEE whenever we cantered. That what told me she was ready for her own steed