With all the information now given, this sounds okay to me.
OP you were always going to get nonsense answers from people who think their children should be treated like children until at least their mid-20s, and that having or even looking after a baby anything below 35 is "too young".
Some people seem to live on a different planet these days, infantilising university students and other young people, and I can't help but feel this is detrimental to those young people and society as a whole. I and a lot of my friends, and now my children who are young adults, do not recognise this as the default and you don't need to go back many generations to find much higher levels of young people of 19 with responsible jobs (having left school at 14, 15 or 16), getting married, setting up home, starting to have children, and even if not that yet (girls especially) still having looked after other people's children for years already. Humans didn't inherently become less competent suddenly! (If brought up to be so that is, and taking different rates of maturing into account of course.)
Although it is the specifics of this case, and not 'any 19 yo who has done a bit of babysitting and seems fairly responsible' that I am in support of, to be clear. The DN is mature and experienced with this child, has support, and the toddler is used to being left with her and the wider family.