Do not stop offering the formula but do offer other foods as well (while still feeding formula-- like, place the other food in bowls etc)-- you want to do abundance weaning, not forced. It is not good that you got these guys unweaned....there is no purpose and it doesn't improve your bond with the parrot long-term.. In many cases, it actually harms it.
http://www.parrotforums.com/breeding-raising-parrots/74363-so-you-bought-unweaned-baby.html <--the is 100% spot on (see silversage's advice)
https://birdtricksstore.com/blogs/birdtricks-blog/why-weaning-a-parrot-properly-is-so-important <- dangers of unweaned baby and importance of abundance weaning
https://www.parrottalk.com/bbbountyfullweaning.htm <- abundance weaning
https://hari.ca/hari/research-facil...cine-pediatrics-housing-feeding-baby-parrots/ <-- PLEASE READ *COVERS HEALTH ISSUES *, procedures etc
I hope you have a thermometer and are keeping conditions sterile in terms of syringes etc...The temp needs to be VERY specific and conditions must be sanitary (for instance, if you touch the fridge handle and then start touching their mix etc, that's not a good idea unless it's all sanitized). Note-- household cleaners are toxic to parrots due to their very very sensitive respiratory systems--so you should not be using anything like lysol or windex or bleach to clean because most things that have a scent *and even those that do not, such as teflon/ptfe/pfoa/pfcs can be deadly*
What formula are you feeding?