You need to lower the water level to about half that. Place a basking spot on one side. Keep the basking light over this with a something around a 25 watt bulb. Check the temps. It should measure 85-90 degrees. The basking area needs to be something they can easily crawl on to that is completely out of the water. Feeding pellets is fine but I would make sure it is Reptomin pellets. They are way too young to sex right now. Keep in mind these guys get huge over a period of 4-5 years if feed correctly. My girl is 11 years old and is 12" shell length and just over 4 lbs. I raised her from a tiny hatchling They will not be able to live together if there are males(which most likely are) as they become quiet aggressive and will terrorize the others turtles sometimes even biting off body parts. You don't have to be in any huge hurry but I would find homes for some of them. Are you sure they are RES instead of Painted turtles? I would definately get a larger enclosure ASAP. Something like a 20 gallon makes maintaining temps easier. You don't want to cook them. Lastly, make sure you have a UVB source. I highly recommend the Reptisun 5.0. These can be bought online as they are expensive at petstores. This should be replaced every six months and can extend over the entire tank.
This is CRUCIAL for hatchlings!

Hope this helps. I have attached a couple of pics. The first if her outdoor pond and the second is her in her aquarium (120 gallon)in winter.