I need behavior help with my 2 year old daughter and almost 4 year old son..?

okay first my son , he's going thru this stage where i'll tell him not to do something & then he'll do it while im looking right at him , but then when i yell at him or put him in time out he reacts like he's confused which upsets me even more . Now my daughter , she screams all the time , i mean a blooding curdling ear piercing screams , i've tried tapping her in the mouth and time out to get her to stop and she just wont . i need HELP im at my wits end !