I had 20mg of lisinopril this mourning and i might have taken the same tonight around 7pm?

BP does not change much for 20 and 40 mg dose so I think you should be fine tonight. Before the next dose tomorrow, check your blood pressure, you can take the usual dose if systolic pressure remains above 120. If it is lower 120, you check BP at noon and then 4 PM, take the pill whenever it is above 120. If you do not have a BP machine, you can resume the pill as usual if you feel fine tomorrow.

Lay down and then get up, if you get dizzy or weak it suggests that you did double up the dose accidentally and need to increase fluid and salt intake to compensate. The exercise should be repeated in the morning and dose of medication will need to be adjusted based on what is occurring at that time. Go to urgent care if in doubt.

For maximum efficacy and minimal risk of toxicity all medications should be taken exactly as prescribed, especially cardiac medications. Specific information as it pertains to the 13 year old requires contacting the prescribing physician, the practitioner that knows the child's physiology best.