Gave my 4 year old allergex loratadine 5ml this mornig at 6:30, and absent minded gave again tonight at 8:30, only afterwards realised should be once a day, is it dangerous and what should i do?

Not a problem: This particular drug has a wide safety margin within the dose. Doubling does not produce any significant risk of side effects like you will see in many antihistamines. Doubling also doesn't provide any added benefit. You may want to place a note or calendar on your refrigerator or medicine cabinet & write down/check off when you give meds.