@Salemtrust just a few thoughts on your update..
The emergency involves a single parent family members younger children and a medical emergency that no one else can help with.
That doesn't sound to me like a serious enough emergency to leave a group of young children that you are responsible for. You still havent specified exactly what you were needed for here - was it that you were needed at the hospital or to look after other children left behind? I'm not sure how you thought it was ok to abandon your childcare responsibilities for one group of children to transfer them onto another group of children...As PPs have said, your response to relative should have been 'so sorry, I have come away for the week and currently have a group of young girls in my care, I can't leave them.' Its difficult to believe that you genuinely thought any other response was ok.
This is my side of the family and a sensitive situations and I wouldn't expect or ask dsd to go and help instead. She's more than capable and comfortable with looking after her sister and her friends though which she has done before with different friends.
But its not about what your DSD is comfortable with, its about what the parents of the children are comfortable with and you can't make that decision for them. You arranged for your DSD to take over without the consent of the parents, then put it to them as something that was happening regardless of whether they were OK with it or not and if they didn't like it, they can either make a long journey at short notice, which depending on their circumstances could be inconvenient or even impossible in all sorts of ways, or consent to having their child plonked on public transport alone for an equally long journey. Those are not acceptable alternatives to a couple of random 23 year olds looking after them.
Also, I fail to see how DSD is not suitable to go help with young children where their mother will presumably be present, but she IS suitable to look after a group of children without their parents being there. Are you saying she would be too immature to help with the family emergency? Out of her depth? Why is she then mature enough and comfortable with being responsible for the safety of a group of very young girls, only one of whom is her relative and with no other parents or adults present? Your reasoning and judgement is just batshit.
If any of the children were distressed I would arrange for them to travel home either by public transport
Please please please stop mentioning the public transport offer like it was a reasonable option. Plenty of people have now told you that offering to put a young child on public transport by themselves for a journey they have never done before and one that is long and far away from home is NOT a sensible or acceptable solution to the problem you created. Can you really not see that?