Is it ok to say, "the child sees THAT man", that's accusative right?
Yes, it is ok and yes, it is accusative. :)
der and den?
Why den and not der?
Because “sehen” is a transitive verb and its object (der Mann) has to be in accusative. Accusative of “der” is “den”.
why "der Mann" has to ben in accusative?
Because it is an object of a transitive verb (sehen).
Oughtn't "The kid is seeing the man" also to be accepted?
Well, to be seeing someone wouldn’t be “sehen” in German.
"Seeing someone" implies meeting up with someone or dating someone in English.