If your daughters have not filed 2019 return and just 2018 return. You could just have them file separately on their own for 2019. You wouldn't be claiming them as dependent for the year 2019 if you would want them to get the stimulus check of $1,200.
If you have already filed 2019 and claimed them as a dependent, you probably won't get any credit back due to their age. You could do an amendment and remove them but just remember, your Child tax credit or Earned income credit(which I doubt if there is any since you claimed 0 income) might have been higher due to your status being changed to head of household and claiming them. The child credit for you was probably $500 but I would double check and see your youngest daughter age at the time you filed, it could have been $1,400(or $2,000 full if you had any tax liability).
If you decide to amend your return, it could take up to 8 to 12 weeks(which might be around the time when these checks are being sent out). And keep in mind, you would have to do another return for your daughters as independent so they can get $1,200 stimulus check, you would have to amend your taxes and then wait few weeks to file your daughters in order to not get an IRS audit letter.
Hope this helps.
