I have been told that one should not stand or walk under trees after Maghrib because of the presence of jinns on the trees.

Answer

It is not a teaching of the Shariah that one may not walk under trees after Maghrib. Jinn are all over the place, not only on trees. Jinn are in desolate places. If someone had an encounter with a jinn under a tree, it does not follow that there is a jinn on or under every tree just as it is not necessary for a jinn to be in every house because in some houses jinn were found. After Maghrib there is a preponderance of evil forces - shayaateen, evil jinn and perhaps other ghoulish creations of Allah Ta'ala. Therefore the Hadith orders us, especially children and even domesticated animals, to be indoors during this time. But this is the 'best' time for fussaaq and fujjaar for indulgence in filth and immorality (fisq and fujoor), and satanic merrymaking.