An undertank heater under about one third of the container..control the temperature with a thermostat, rheostat or dimmer switch. The melting point of plastic is high enough that an undertank heater working properly and controlled with a thermostat (or set on low if a human heating pad) won't melt it.
Overally the setup for babies is the same as it is for adults, only difference is the size of the cage. I have used 12"x6"x6" (lxwxh) plastic containers for baby corn snakes for up to a year of age (depending on growth rate) with about a third of the length over a human heating pad set on low (temp over the heating pad stays around 90F or a bit less, depending on room temperature) with a paper towel tube cut in half (length wise) and running the length of the container for a hide and just a few layers of paper towel for substrate. A small ceramic dish (about 2" diameter and 1 1/2" high) half full with water, on the cool end of the container.
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PHLdyPayne