Hi ronaldo60, Its hard to say without photos but leaf rot could be due to overwatering. Sometimes with bananas the older leaves do tend to dry up and new ones in the center eventually replace them. However, if they feel mushy or are dying but are not dry it may be a root rotting problem.
Even though you have them indoors with some heat and light, their growth rates may have slowed because the heat/light is not as much as it was outdoors. So they could be staying wetter down by the root areas. I know some tropical plants like monstera deliciosa will sweat when they are too wet, so it could be for bananas too. After checking out the water situation you could also try adding more perlite to your peat mix. Peat does tend to hold alot of water and the perlite with help with drainage. (You can always change the mix again if/when you need to repot in the spring/summer and they go back outdoors). I would try to bring the temperatures up for the young plants. I think 40s at night are probably too cold for these babies. If you get some heat in there, they will probably grow faster too. Other things to consider once the heat and root situation is fixed is you may wanna try misting the leaves a few times a day especially if the heater is on. If your oplants were larger I would say just let them go dormant but seedlings would probably die for sure. Good luck
~SJN

If the leaves are drying, then it can be lack of humidity.If the leaves are rotting, then i agree it may be root rot problem. You may want to cut back on watering a bit,but you can mist the leaves with a spray bottle to raise humidity.

Thank you both very much for the feed back. As the leaves are kinda soggy when the rot back i will lighten up on the water, i may try and repot them in ceramic pots with plenty of perlite and drainage, that way they can sit directly under the light and not get whacked by the kids.

Many thanks,

R,