I think you are describing a power converter. I have never had one that hums, but I have heard that it does happen. If you let your kids stay in the trailer by themselves, make sure the propane tanks are turned off. Also, just in case the WH is gas and electric (they do make them, it could have been added aftermarket), go open up the access door to the WH and there should be a blue knob in there. Make sure the knob is turned to the "OFF" position, NOT "ON".
Most of all, make sure the batteries in the smoke detectors are new, and you may want to consider adding a CO2 and LPG detector in the future.
If you are still uncomfortable with the trailer, especially if you haven't taken it camping yet, there is always the option of unplugging it and wrapping the kids up in extra sleeping bags and blankets.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR