Why do you need flex?
having had both setups, the flex is great – it massively reduces snatching and makes pulling away (especially uphill) a load easier (as you can tension the line and set off together, using the spring to get both of you over top-dead centre as you’re going walking pace which is the time the kids most likely to fall). it doesn’t twang about, but gives a bit (maybe growing a foot or 2 from un-stretched length) making the whole experience way smoother.
for a more advanced passenger, or for smoother trails a static/retractable line is probably fine, but for younger kids the more you can do to make things easier for yourself the better (I tend to have one kid on the wee-ride and one being towed..)