I recently went shooting with my daughter and SiL, he insisted that she shoot his 12ga o/u.
She struggled with it but was hitting clays, seemed to enjoy the game and recoil didn't seem to bother her too much but it was obvious both his and my gun were just too big, too long, & too heavy for her. Her swing was just awkward.
I suggested we go back to the club office and see what rentals they had, suggested he ask about a gas 20 ga in a youth model and offered to pay the rental fee. He comes back with a Beretta 20ga 686 Silver Pigeon and a couple 9f boxes of shells, absolutely beautiful gun. She was shooting doubles consistently in minutes (easy course) and her face just lit up. Her swing became much more natural and general handling was obviously much more comfortable to her. Even so I noticed the recoil was pretty sharp given the lightweight gun. My SiL was just astounded at her ease in handling the gun compared to how awkward she had been earlier.
I then suggested he start looking for a 24" - 26" gas gun, and pointed him to the Mossberg SA-20 bantam model as a decent shotgun for her w/o breaking the bank. The problem with an O/U in a really light frame gun is the recoil can still bite a novice shooter.
I just went through the same exercise with my wife, as bad as I wanted to buy her a nice 20ga O/U, I know she will enjoy the 20 ga gas gun I bought her way more. If she turns into an avid clays shooter we'll upgrade to whatever she wants but I suspect she'll tag along when we do family shoots or when I go solo as she enjoys shooting but she is never going to shoot a 30" 12 ga. competition O/U.
All that to say buy something that fits them now, if you find something with extra spacers, longer butt stock or an upgrade program in the future great. I think shotguns in particular need to really fit stature (length, LOP & weight) well since for the most part they are always shouldered and in motion. Good luck!