On the day they're born, balloon molly fry are about 4-5mm in length; on occasion you might get slightly larger individuals, who are born at around 6mm. They grow very quickly for the first few weeks, if they have enough food and space. Based on your photos, I would estimate that your fry is likely anywhere from 3 to 5 weeks at most. If you can get an accurate measurement for its length, I may be able to narrow down the age estimation a bit.
I've had good results with feeding Liqufry the first few days, then transitioning to powdered flakes (I just grind up regular flakes with a small mortar/pestle set; you can buy powdered fry food, but it's quite expensive compared to just "making" your own), and moving to a combination of flakes and frozen foods when they're large enough. Even when they're too small, I may drop in a tiny frozen brine shrimp, so that they can get a taste for it. I typically feed young fry at least 3 times daily.
If you can get a small tank for him/her, that would be great! The fry will likely be fine in the trap for a few weeks, but the more space you can give it, the better. A 5 or 10 gallon tank will work well until it's large enough to move in with adult fish.