Uuugh... Not another one fall into "I should hide it so they don't think i'm weird" train of thought
What do you think will happen when in the future someone found out you have more magic than normal cause you keep fainting from overusing it! They will think you are DEFINITELY a noble or someone special.
You are a kid dam it! Make use of that fact! A kid should be naive! You could just proudly show them you could use light after seeing the picture in the book or "accidentally" tell them how you could use more light every time you woke up after passing out. At least that way you could work together with your parent on the story or on what to do in case it got found out later since they knew about it already.
They should understand and even worry for you since a parent who lost a child WILL NOT want to lose another one. Thrust them dam it! And whats more, they are an experienced parent. Unless they don't care much about their child or aren't around much to see you, they would KNOW when the kid is lying. Just... Be truthful... It's will make things easier in the future.