Lychee seedlings need warmth and humidity, and the right amount of sunlight. Setting them inside near a window may not be quite enough light for them. Lychee seedlings are not like houseplants!

However, if the humidity is not high, full sun will also be too much for them. They do good under a partially shaded patio, but again, the young seedlings cannot handle cold.

Realistically, you probably need the right equipment to grow lychee seedlings with any chance of long-term survival. A little greenhouse, with a heating pad under a bowl left filled with water. Or maybe grow lights.