I heard that internal use of essential oils can cause seizures in kids. Is it true ? If true then what is the reason behind that ?

Yes it is! I wouldn't even use essential oils internally on myself, let alone a child. Essential oils are basically really concentrated distilled volatile aromatic constituents of a plant. A drop of essential oil is equivalent to 15-40 cups of medicinal tea. No one would want to drink 40 cups of tea in one sitting, that is madness. Internal use of essential oils have been known to cause seizures because it is a neurotoxin to kids.