You're likely overthinking it. Did you weigh the bikes? Hold them while you step on the scale, then subtract the weight. How big and what type of MTBs? What size of kids bike? 30, 35-poundish bikes - on the heavy side - would be around 100lbs. If the manual says 200lbs, the engineers probably designed it for 400lbs. Not that I would exceed the specified limit, but going up to what the manual says should be OK. Yeah, there's rust and wear, but just check for it once in a while. How many bolts are holding the hitch? Unlikely that all of them will shear off at once.
And just for peace of mind, load bikes on the hitch rack from heaviest to lightest. Lever arm/torque and all...