'Lambaste' is stronger than 'criticize'. It seems out of place in a sentence about a toddler. In that sentence You'd be more likely to use 'scold' or 'reprimand'.

'Lambast' is an extremely strong verb. It wouldn't be used in either of your examples. It means to criticise something or someone extremely strongly or harshly. It just isn't an appropriate verb for the situations you describe. Being forgetful isn't (normally) something that would cause someone to lambast the doctor. And you certainly wouldn't lambast a small child, even if they were misbehaving (other more appropriate verbs to use would be 'to tell off' or 'to scold.')