One of these, at Walmart, under $20.
Patton Electric Utility Milkhouse Heater, PUH680-WM1 - Walmart.com
They are inexpensive, indestructible and cheap. Fan is a bit noisy. They have an amazing ability to keep the unoccupied/stored trailer about 40 deg F. when it's below freezing. Some heaters will not "idle" that low. We have stretches of weather here in Georgia where the camping is cool but nice and then a sub-freezing cold snap comes along and you have to winterize and de-winterize to camp a few days latter. Or just plug in the milk house heater for a night or two.
For camping just turn up the thermostat and you're toasty. If not used in awhile plug it in outside for a few minutes to get rid of the smoky/dusty smell.
We don't have little ones. If we did we would use the more expensive, larger and heaver oil filled heater to prevent burns. They also run silently.
In either case we sometimes run a heavy extension cord out the slide seal to the pedestal so we don't trip breakers trying to heat and make coffee at the same time.