The potentially lethal dose (with desipramine, imipramine, or amitriptyline) is as low as 15 mg/kg
https://emedicine.medscape.com/article/819204-
clinical
Doses of less than 20 mg/kg are unlikely to be fatal or cause severe complications but individual variation in absorption, protein binding and metabolism limit any meaningful prediction.
https://emj.bmj.com/content/18/4/236
A 1974 level 4 report of 60 children hospitalized for a one-time TCA poisoning estimated the minimum lethal dose of amitriptyline or imipramine to be 30 mg/kg, although the authors included the case of a 2½- year-old child who died within 4 hours of ingesting 15 mg/ kg.. Life-threatening symptoms in adults are often seen with TCA doses in excess of 1000 mg.
https://www.tandfonline.com/doi/pdf/10.1080/15563650701226192