Still unclear about type of frame.
Are you are willing to have at least one curved adjustable horizontal cross-bar?
It will help keep the frame's rails, or sidebars, in the position you want for it to function at its best in holding the shape of the 'envelope' and will help keep the pack contents from bulging or poking against the wearer's back.
The adjustment doesn't have to be fancy. It could be just a couple plastic hose or rubbery polymer kite T-fittings that can be moved up as the kids grow. You would not want too tight a fit on rubbery polymer fittings, as they would then not be able to be moved once the lubricant you used installing them dries out. (Mineral spirits work well). The Ts can be cut shorter at the top for easier movement, or bored slightly for a less tight fit. Worst case, you could cut the fittings off and replace them - they are inexpensive - buy some extras. If you are using larger tubes for your frame rails, you might want to look at PVC plumbing T-fittings, for some that fit with just enough friction to stay put until you want to move them. These can be bored also. With larger tubes you could used tiny screws or pop rivets at the outside to hold the fittings in place - not a good idea with tent pole size tubes whose integrity would be destroyed by that.
Then you would create a series of horizontal slots, running directly on top of each other, around shoulder height in the center of the backpanel, reinforced with heavier material bonded and sewn on. In the old days, it was a leather patch. Nowadays it could be a plastic sew-on patch, or even Zpacks 2.92 oz cuben/nylon laminate if you want to get fancy. You can also 11use two such patches, side-by-side, to precisely locate the width between the shoulder straps at their top. The webbing exiting the top of each shoulder strap goes into the slot, under and around the cross-bar, back out through the slot, and is secured with a small buckle, also attached to the top of the strap where the webbing comes out. The excess strap coming out of the buckle can be tucked into the slot to keep it out of the way. You may want to slide a short collar onto the cross bar to keep the straps separated inside the envlope at a predetermined width. Allow enough width for the kids' growing.
As the kids grow, you remove the frame, raise the cross-bar, reinstall it, and reinstall the straps through a higher slot. Voila!
Without knowing more about your frame, that's my best proposal.
Edited later for to add info.