Unc - two other things I forgot to mention in my post above, which may or may not apply depending upon the age of shooter for which you are cutting this stock...
1) Small bodies typically come attached to small faces, so a cheek riser is also often needed for youth stocks. I use Victor Company universal cheek risers because they are cheap, yet well made, adjustable, and look clean and intentional. Polymer stocks are typically thick enough to allow these to be secured with wood screws, but an ambitious fellow pursuing the LOP reduction may also decide to lay the stock on its top and pour a footing and sink threaded anchors (I have a thread here on this process in a fiberglass/carbon fiber stock from ~2yrs ago). This helps ensure the riser could be removed and reinstalled almost unlimited times without worry of cross threading and chewing up the thread hole as might happen with repeated removal and replacement of wood screws directly in the soft polymer. A bit of foam rubber formed to fit the curve of the comb (beveled on the edges to taper to flush with the sidewall and bent to a trough which fits the comb, and covered in duct tape while bent,) and held to the stock with an Allen Cartridge cuff works well too for a more garage expedient option.
2) I typically use the common LOP measurement method of measuring from the pit of the elbow to the pad of a bent trigger finger, with an angled wrist roughly angled the same as whichever pistol grip you might choose... and then add an inch. I’ll often tape a roll of quarters to a hammer handle, then have the kid hold the hammer angled forward with their 3 fingers below the roll of quarters, and their trigger finger reaching forward around the wrapped coins, with the hammer handle angled similar to the pistol grip of the stock. For 2-5yr old kids, I often find 8.5” fits well, then 6-9, maybe 10yr old kids, 10-11” fits well. They’ll stretch a little in the earlier ages and be crowded a little in the oldest of the span, but overall these seem to fit most “average” size kids in these age brackets. Certainly not as poorly fit as the 11.25-11.75” stocks on common youth model rifles which simply remain too long for the young kids which might actually find them useful. Cutting a stock on a “lifelong model” is a much better path.