Sweetheart, I know exactly what it's like having to live from one week to the next counting every single penny just to put food on the table for my babies.... BUT hunni, you have to be proactive in getting things sorted. You need financial support. If he's going to go on a stag weekend then he can pay to feed his babies and for the new uniform. Those are essentials, going and drinking the money that could have been used for that is unacceptable. You wouldn't be able to do so, so why does he?
I'm also wondering why you protect him and why you don't hold him accountable for any negatives. Being a father is generally easy, but being a Dad is so much harder. It's about standing up and putting your babies first. Our children are now 22, 19 and nearly 18. There have been points when my husband was having a breakdown when he wasn't really involved as much with the day-to-day stuff, but he still was there for those events such as performances or parents' evenings... He still feels guilty for not being there when they were little, but they don't remember that. I wonder how your ex feels about the time he's been absent or not provided for them - whether that's through his own earnings or through benefits/organised maintenance payments - you can't choose to provide for them or go away for (what could be) an expensive weekend away...

I know you say that he hasn't got much, but love what do you have/are able to pay for that isn't a luxury, but would cost the same as this stag weekend - he isn't going to have been paid for, drinks and all meals paid for by the groom..... Each pint will add up love.

Sweetheart, do you see how unfair and wrong he is? With the way he's treated you in the past, etc and you are constantly putting his wants above your children and putting you at the bottom of the list. Love, counselling is far more important than going away and drinking...
I know you're going to read my reply and think I'm being harsh and unfair. That may be true, but I can only say it as I see it - that and I've always promised to be honest and tell the truth as to what I feel from replies.
Hunni, you need to try to put yourself first love, you are so so important and loved and wanted by your little ones... I wonder what you'd say if it was one of them being treated like this? Or what if it was me or one of your close friends?