First. Do a parenting course. It helps the diagnostic process. It may help with behaviour it might not. But it helps you with your confidence. You can say at assessment I did X course and using yz strategies but they are not working with my children.
Second.
Think differently about what is achievable. You can't do the same things with autistic children as NT children. Don't do a lot. Very short visits lots of down time. (Unless you have a sensory seeker)
Third. Prevention.
Keep well fed, well hydrated and well rested. Reduce the stimulation.
Google occupational therapy and exercises for autistic kids. Do every day.
Consequences: mine lost a privilege but could earn it back by being (desired behaviour) if couldn't earn it back then there was no point being good for the rest of the day.
Try deep pressure (firm hugs) when not behaving. It helps calm many of them. Also rock in one direction. I perfected a way to hold them that restrained them safely during a meltdown. Not before I was head butted in the race and scratched across the face. And hair pulled.
Keep kids separate until fed breakfast or tea.
Use screen time. Mine finds it mostly calming as do I. (Autistic) pick the game/video carefully though don't uses something they can loose at and then meltdown.
Try sticking one in front of the washing machine to watch that. That worked for one of mine.
Work out what they are sensitive to. They may be over sensitive or under sensitive.
Jumping off stuff may be beneficial.
You can get Sen reins which are stronger if they are runners.
Practice naming emotions with them.
Read the positive behaviour strategy posts above. They work reasonably well. (They are still autistic so will still.meltdown.and you will still get pissed off sometimes and grump at them, because you are only human.)
Diet seems to be something that might work but it depends on your kids restrictions. Plenty of fruit and veg of the sort they will eat in the form they will eat it in. Reduce the processed stuff. (One of my autistic teens eats lots of processed stuff so I know it's not always possible.)