Witholding of removal and traveling

I was granted withholding of removal last year. I was able to travel from NY to FL or California without any problems. Can I travel to Puerto Rico on my DL too? How about US Virgin Islands? Also is there a chance to reopen the case and be approved for work sponsorship or I need to wait for my kids to reach 21?
As long as the flights to PR or the VI are direct and non-stop from the US mainland, you shouldn't have any problems. However, make sure you have photocopies of your withholding of removal documents, your valid EAD, and I would even say a document from your attorney explaining your status. CBP is still known to conduct random checks on both places due to illegal immigration from neighboring islands. Do not take any boat tours or excursions anywhere, especially to any other islands. If you do that, you could risk a CBP check assuming you "left" the country and are trying to "re-enter". Another thing to consider is that in the off 1% chance your flight has to make an emergency landing on any other island for whatever reason, the flight can change from domestic to international. There's a number of factors that go into this. If the flight changes status, upon landing back in the US, you'll have to be able to prove you have the legal right to ENTER the country, which you do not. You will likely be put into removal proceedings again and could be deported. These are all very tiny risks, but you should know them. Consult with an attorney.

You can possibly obtain a green card through employment. But it's going to be tricky because at any time you have withholding of removal, and you try to obtain any other immigration benefit, your original WoR is re-opened. If any circumstances have changed that granted you the WoR, you could be deported instead of getting the green card. Consult with an attorney if you find a job that's willing to sponsor you.

Your children being able to sponsor you at 21 is very dependent on a lot of different factors. May I ask, did you enter the US with inspection (legally)?