Question: Often, non-Torah-mandated holidays are moved to avoid desecration of Shabbat. Why not Lag B'Omer? There will definitely be kids lighting bonfires well before Shabbat ends.
Answer: The problem is that it is not exactly a holiday with mitzvot but a popular custom. In Shut Sha'arei Tzion (#14), Ha-Rav Shmuel Rabinowitz - Rav of Kotel and Holy Places in Israel - discusses this question regarding the celebrations at Kever Rashbi on Meron and he says that the bonfires should be started later in the night and this is the custom of the Admor of Boyan, but he also refers to a letter from Ha-Rav Zalman Nechmiah Goldberg (Rav Shlomo Zalman Auerbach's son-in-law) that the bonfires should be done during the day for the reasons you mentioned. The Chief Rabbi of Israel, Ha-Rav Yonah Metzger, also called for the bonfires to be lit of Sunday during the day.