My answer to this question is only my opinion, but I think the answer depends on the age of the children, the age of your daughter, and whether your adopted children have been with you for some time or have only been recently placed with you. It may also depend on how many children are involved. If your daughter is an adult it would be legal, provided the adoption has been finalized (which would mean you have a legal right to choose a capable, adult, care-giver while you work. If your daughter is under the age of 18 I don't know if it would be considered, "illegal", to leave the children; but there's the chance it would be considered, "negligent" or potentially endangering the children, depending on everyone's ages, how many children there are, how long you'd be gone, etc.
Adopted children who have been with you for some time are like any other children, so whether or not they're adopted shouldn't matter in many ways. I'm guessing if the adoption is recent enough that social workers will be following up on how things are going, they would not approve of your leaving the children.
In general, if the adoption has been finalized and provided the situation wouldn't be considered "illegal" for any children (adopted or not), it would be legal.
One other matter to consider is whether there is any court order or other arrangement in place that would mean there are some things you have agreed not to do (like leaving the children for a long period of time). I don't know how likely it is you'd have such an arrangement, but I do know that in some open adoptions parents work out their own arrangements (with the "blessing" of a court) as part of the adoption process.
Another consideration is what your state laws are regarding full-time care-giving, and what would constitute a "day-care" situation. Not long ago a woman in the US got in trouble for letting a few of her neighbors' children come to her house for a couple of hours after school (no money - just a favor). The state's day-care provider laws were the issue, even though it seemed ridiculous to most people. In other words, it wasn't legal in that state for a friend to watch a few neighbor-children for a few hours a week. Your daughter, of course, would be considered your children's sibling, but you may want to double-check if the number of children and/or time involved might be enough for your state to call it a "day-care" situation.