My child won’t stop lying :(

I’m loosing it! My child is making me so angry lately that I cry! He’s 6 years old but everyday he lies about something! Today my two year old was screaming and crying in the next room while I was trying to settle my baby down so I went in their and he’s rubbing his head the two year old so I said what’s going on so my 6 year old said he tripped over my knee... so as I was about to leav it my two year old said mummy (my sons name) hit me so I asked again and he said he smacked him with his hands to use them as swords, I said no your just being spiteful you have toys to use as pretends swords but no you chose to hit... the fact is he lied to me. Yesterday I was doing laundry and after my two year old has rather a slice and a half of bread (bearing in mind my kids had just had breakfast no longer then an hour ago) my 6 year old then comes to tell me so I told my two year old off put him on the corner, I asked why didn’t you take it off him he was silent, I then said at the point I realised theirs a slice and a half gone you must of saw him I asked did you watch him do it? He said no, I asked are you lying? He said no as soon as dad got home from work he asked and he was strait up honest he just stood their and watched... I’m losing my mind with the lying :( and I’m exhausted with how my 6 year old behave the fact he’s lying and hurting baby’s and toddlers and he just has nothing to say about his actions he goes to a mute I don’t know what to do I’m literally beating myself up about it!

UPDATE- my other two children are 2 coming on 3 and a 13 month old, I don’t think it adjustment as it shouldn’t take this long to adjust he’s behaviour has been going on for 3 years but I’m scared to get professionals involved:(

UPDATE- I have tried several kinds of punishment, I’ve taken toys/tech away from him, I’ve trued a smack on the bum and bum only as I wouldn’t go further due to my upbringing was rough, I’m trying stand in the corner, send him to bed early after dinner so he can hear the other two play so he may think about what he’s done. If anyone has suggestions please advise me.

UPDATE- it’s not the fact of the not taking bread away even though he has took things off him before he shouldn’t have and gave it too me, it the fact that he stood their and watched him instead of telling me and the fact he is lying every day it’s not once or twice a week it’s an everyday thing their is a problem then I feel. And hitting baby’s and toddlers theirs a problem their defiantly

UPDATE- the reason I don’t want professionals involved is for two reasons, one is that because I was in care and had my son at a young age social were involved at 2 years ago I got them off my back I don’t want to be a social service family I just got my kids being mine again and two I suffer with a mental health problem myself and I don’t want it to be a link thing when I know I wasn’t like that myself growing up I got the mental health problem from a bad upbringing and that I’ve always avoided for my children yet it’s not good enough.