I understand how stressful this is OP, I've been in your shoes. What's even worse is it sounds like you've been everywhere asking for help, so CAHMS, school and so on, and got nowhere. The MASH safeguarding referral might be an opportunity to restate these needs. They will just talk to your children alone to see if they have anything they want to say without a parent around. That part is not against you, it's for them to raise anything.
I have to say having been through this a couple of times, we never even had a social worker allocated, they just dismissed the case over the phone as we had a private therapist at the time for my dd and they just washed their hands of it and ran away fast.
Use this horrible event to pressure the GP and services to refer again to CAHMS and/or move up the CAHMS list.
I didn't get whether you are a lone parent, I am, and that makes everything exhausting and so tiring, you feel you are the only thing preventing your dc2 taking their life, and you simply can't watch them 24/7. There are no good answers and the system is absolutely bare and will not help you. My main advice would be to try to get your dc2 some counselling either privately or on a reduced rate, and put that before more or less anything other than essential bills, as the state won't step in. If they are a carer for your son (even in minor ways) then they may also get counselling through being a Young Carer.
Speak with the SW, don't be hostile, let them know you are desperate and will do anything to help your children, and use this as a way to leverage more help for you and for them.