Hello
It's not possible to make a definitive diagnosis from an image, but here are some questions/suggestions.... Have these seedlings been exposed to sudden changes in light/temperature? Some of the damage looks possibly like burning caused by a bit too sudden hardening off or placed too close to a window pane that had direct sun and/or too cold temps. 

The other worry I'd have, although these are very small plants, would be a fungus. Tomatoes outdoors are susceptible to several forms of blight. It would be odd to see this on indoor-grown seedlings but some of the leaf damage looks somewhat like blight. Was the potting mix you used sterile new mix or did it contain garden soil?

Although the growing point on the one plant looks ok, even the leaves near the top are faded (chlorotic), this may indicate a problem with nutrients or pH level. Have you fertilized these guys yet? Sterile potting mix is fine for starting seeds, but contains little nutrient value, so a dilute fertilization every week or so is a good idea at least be the time the seedlings have their second second of true leaves.

If you used sterile potting mix, make sure to give them some fertilizer. Remove the damaged leaves and dispose in the garbage (not your compost pile). Bring them gradually into full outdoor conditions over 10 days or so. They may be fine. If they continue to have faded damage leaves, it is best to destroy them and start again.

Glen - THANK YOU for the very thorough answer and for sharing your knowledge.

I have had them under grow lights and near a pretty sunny south-facing window since they germinated, so maybe I need to take the lights away now that it's getting sunnier. We have also turned the heat off in the house so it's not as constant as it was when they were smaller seedlings. I will try to regulate the temperature again.

I did use sterile new potting mix, but have not fertilized yet. I will try that as well. 

Thank you again! I learned so much from you!