These roselle seeds were germinated in a 15 inch (please don't bang your keyboards in anger) soil from the ground with cocopeat, compost, vermiculite and perlite mixed with it. Didn't knew the dangers of soil back then. The third and fourth leaves came out yellowish which I suspect was due to soil compaction. So I have repotted these in 3 inch plastic pots with fresh potting soil. Where should I keep this (in shade, in some sun or in full sun?) and for how much time? I live in Delhi, India and the weather's quite sunny with temperatures touching 40 degrees. Three of the second seedlings' leaves have been plucked off by a neighbour's toddler.
Keep them out of hot direct sunlight until they are bigger and more robust - a little early morning or evening sun should do no harm. When they are bigger, you will likely need to pot them up into something larger, and exposure to sunlight should be reasonably gradual (not in midday sun for the first week or so) to give them time to acclimatise.
Delhi has a pretty humid climate. So in addition to giving it filtered sunlight (placing it under a decently dense canopy of a tree), also check the water levels. Don't underwater, but also don't over water the tree.
To add on to what @Bamboo said, you'll have to play it by ear. If the leaves are wilting and the soil has decent moisture, it probably needs more sun. If the leaves are turning brown/brittle or you are seeing sun scorch on the leaves (black/red burn spots) on the middle of the leaf, then it needs less sun.
But again, regardless of where you place it, make sure the water levels are right for the plant. Both underwatering and overwatering will kill the tree.