We share bike budgets.....though I think yours appears healthier than mine. Can relate with the kids, but fortunately the wife supports their passion, i have to fend for myself. Surely you can capitalise on the growing kids and pick up the scraps? That's how i survive mainly. Point out to her that you're actually SAVING her money using a second bike with cheaper replacement parts, rather than wearing out your weight weenie race parts.
I bought the Turner for exactly that purpose when I cracked the first Procaliber, then life happened. Mrs George did point out that when we picked up the blue one.
I'm in the fortunate position of having my main job pay the weekly expenses and a side hustle that solely supplies the funds for wheel related expenses, either bike or car, and other unexpected disasters like water pumps dying. While it chips in for some stuff, it's not relied upon to prop up our weekly house budget. It is a bit seasonal, so I won't have as much to spare over the next couple of months. Mrs George 100% supports their passion, and 100% supports me paying for them
. So the saving her money doesn't come into it. And both the boys only have one Bank of Dad bike owing to them. If they want anything fancier they'll be old enough to pay for it themselves.
Definitely helps that I was riding wanky road bikes when I met Mrs George so she doesn't really expect anything less with the MTBs, and she sees value in buying the best we can reasonably justify for the boys.