No, you cannot have someone "go get your kids."
1. Why is this under Immigration? Are your children in another country than where you reside? You will have to handle immigration matters, but that comes AFTER other legal steps you must take.
2. Who has custody of the kids? Apparently, the baby-mom has them? Is this just a de facto arrangement? Or does she have court-ordered custody, and is it a joint-custody agreement, under an order which includes custody, visitation & child support? What are the terms of the court order?
3. You need a court order of custody. If you want to take those kids to another country, it will need to be a court order of permanent sole custody, or else you will need permission of the other parent to take them to another country. You have to hire an attorney in the city/country where the children reside, contest the mother's fitness to care for the children, and get the court order before you can do anything about visas, etc.
4. IF you are awarded permanent sole custody of the children, then you need to apply for their visas, or else prove they have birthright citizenship in your country of citizenship & residency, and get them passports from your country of citizenship. Did you hold citizenship in this country at the time of each child's birth? Prove you held citizenship in that country at the time of the birth(s), and prove paternity (DNA tests all around). Register birth to citizen while abroad, then apply for their passports.
5. Did you change citizenship (obtain naturalized citizenship) after the births of the children? You must arrange for immigration of minor children under the laws of your current country of citizenship. Are you in the country where you currently reside on a temporary visa (e.g. student, employment)? You must obtain dependent visas under your visa, proving you earn enough to provide for their total support. Or do you hold legal permanent residency status? Again, you need abide by this country's immigration laws to get them visas to enter the country where you are legal permanent resident (e.g. ILR, if you are in UK). Do you hold some other legal status allowing you to stay in the country where you are? See that country's legal visa & immigration laws to determine whether you can bring your children, and what they will need to enter. IF you are not legally present, you cannot bring your children. You are going to need to hire an immigration attorney to handle visa/immigration complications unless you were a citizen of the country at the time children were born & can get them citizenship in your country of citizenship.
6. After you hold court order of legal custody, and obtain their citizenship + passports, or their visa/immigration matters & obtain their visas, you need to go get the children and bring them to the country where you legally reside or hold citizenship.
7. Depending on the country of citizenship & residence of the children, how efficient their court system is, how effective your attorney is, etc., it can take a year or two to get the court order of custody. Then depending on citizenship issues, it can take a year or so to get visas or deal with immigration matters. Expect to spend tens of thousands of dollars on legal fees, court costs, visa/immigration applications, etc. Before you travel IN PERSON, and pick up your kids yourself. Immigration matters will be vastly simplified & very fast ONLY if children have birthright citizenship in your country of citizenship & passports from your country.
Take those kids without court custody order AND any passports/visas required, and you can be charged with kidnapping & have Interpol tracking you around the world. Do it legally, or spend a good many years in prison - without your kids.