They are looking a lot better this morning. They are looking brighter and more alert.
Personally I would at this stage cut down the amount you are feeding at each feed and if you have to feed a little more often but I would also let them completely empty their crop first. By doing this the crop muscle may work a bit better and shrink more to normal and not so stretched. If anything I would give some very watery papaya until the crop starts working properly again. You also didn't say how many times you are feeding them. But usually they should be fed 3 times per day, that is if their crops are working properly. Maybe because of their size 4ml is a bit too much, drop that down to 2 or 3ml, even if you have to feed 4 times per day and make the food runner. Depending on the formula their instructions for thickness of the mix is a bit off, especially for younger birds. Personally I am not too impressed with most of the commercial hand raising mixes as nearly all of them contain soy and I think this is the meal that does harden in the crop when they draw water out from the crop. I am not a fan of soy meal as so much of it has been genetically modified. For this reason a number of years ago I started making my own formula, it does not appear to separate in the crop and is more easily digested by the babies as the main cereals I use in it are partially pre-cooked because they are baby cereals and based mainly on rice. If you want this formula I am perfectly happy to give it to you. But changing formulas half way through raising is not always advised.
As you are in your Summer at the moment, dehydration is a bigger factor than starvation. Baby birds can go at least 36 hours before starvation starts but they cannot go many hours before dehydration. So I would keep them on quite runny food at this time.
Years ago I had to hand raise a baby cockatiel from day 1 and every time I tried to thicken his food slightly his crop would stop. I had to milk his crop about 3 or 4 times every time this happened and basically start from scratch. I didn't think he would ever survive and he was so tiny. I successfully hand raised him but he was much smaller than other babies the same age. By the time he was six months old he had caught up to the other birds in size.
As a nurse are you able to get Hartmann's Solution, if so you can use this to mix with the papaya as opposed to just plain water. You can heat up small quantities in the microwave for mixing with the formula or papaya. This will help to keep them hydrated.
As for how to use a crop needle I am posting a link to Vetafarm Australia. This supplier is run by vets and they have a specialist avian vet with Tony Gestier. They have a range of online videos on various subjects and 2 of them are how to use a crop needle (they also supply them). I am giving the link that has all of their videos as there are more than one is with regards to birds that you may find informative.
Videos - Vetafarm
If the link does not work because you are not in Australia just do a Google search for Vetafarm and go to videos as they also have an international site that I can't access. But the 2 videos on crop needling are also on Facebook.
Just a little thing that I do to with baby birds is that I put a small stuffed toy in the brooder with them, I have a few of them so that I can wash them. But it gives the babies something to snuggle up to like they would with mum and they seem to find some comfort from it.