From the description you have a an Eastern Painted Turtle. I love those guys. They were my first love of the Herp world. When I was a kid I would spend hours in the creeks of Maryland looking for them. They are fun pets and since you live nearby an area where they are from I would see no harm in keeping it.
The 30 gallon is a good size for your turtle for a while. That may last him a few years. The setup you wanna make is pretty simple. Fill the tank about halfway with water and have a log or rocks in there for it to bask on. If you use rocks make sure they are secure and they will not move. I had a baby turtle drown because of that...very tragic. Over your basking area provide a basking light with a ultra-violet light bulb. Have some clean gravel in the bottom of that tank for substrate. I would recommend maybe about an inch thickness and not much more (that would be more to clean). Put places for your turtle to hide at the bottom of the water (rocks, logs, flower pots, etc).
They will feed on earthworms, fish (minnows, guppies), tadpoles, and crayfish. I would recommend feeding it these items so that if you want to return the turtle to the wild it will have an easier transition. You can always buy turtle food at a pet shop. When they get older they will eat more plants life...aquatic plants.
They live for about maybe 40yrs (guess...can't remember exact numbers) and grow to about 4.5in-6in. Can get to about 8in in length. Cool turtles! Have fun with it. I must say you are a cool mom for exposing your kids to herps.
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14 Leopard Geckos 5.9.0
2 Tokay Geckos 2.0.0
1 Graybanded Kingsnake 1.0.0 (on vacation in my house)
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1.0 Ambystoma tigrinum
0.0.1 Leopard Gecko Egg Cookin!
