A meltdown in itself IS an anxious reaction to a situation, but violence should STILL not be tolerated. The OP is saying how her younger DC's are sometimes on the receiving end of the violence - she also has to bear in mind THEIR emotions.
As do I, with 4 DC's and it being DD, the eldest, who is most likely to have violent meltdowns.
Yes, there is a HUGE amount of avoiding triggers done, and a huge amount of trying to jump in before she lashes out, and a lot of redirecting to anger, but that still doesn't change the fact that for my OTHER DC's emotional well being, I have to punish her.
I have had these rules since she was 4yo, now finally at 15 she is sonetimes (NOT always) able to stop and redirect herself to punching the door, or kicking the sofa, instead of her siblings. Work in progress.
And once she is calm, she can accept that the punishment will happen even if the violence WAS her lashing out in a meltdown.
It's taken 11 years for it to stick even a little, so it takes FAR more sustained effort than it does with an NT DC, who WOULD care about losing things that they like at a much younger age, but we are slowly improving.
It's bloody tiring mind you!