@Countrypie firstly apologies that this is going to be a long post. I was adopted and you sound similar to my adoptive mother in some ways. She regretted things and suffered terrible feelings of loss. She also suffered terrible depression though I am not sure whether that is affecting you too.
It sounds as though you need to be able to grieve and come to terms with losses and also get help for your DC for their losses.
In relation to your DC's losses, I think that the very serious risk for them and other children in similar situations is that if there is no love and connection they may develop a deep unshakeable and long lasting belief that they are they are unlovable. They may grow up with literally no grasp of what emotional intimacy looks and feels like. This would affect the relationship choices they make and may affect how they behave in relationships as well as how they feel, and so there is a ripple affect outwards. They may suffer dissonance if what you tell them does not accord with what they pick up on in relation to your feelings, eg if you say of course you love them when you do not, and if so find it hard to access help as an adult as they don't know what exactly they need help with. If any of this is likely I think getting help for them now is urgent, as well as getting help for yourself.
The school sounds lovely but as the DC get older more autonomy is required and they may begin to struggle unless they are helped now - this is what I have seen before in any event. If they compare their lives to those of their friends who they know who are loved, and in whose family the dynamics are completely different, it may make them very jealous and angry, or depressed, which may be well hidden at the moment but show more later.
Much is written about adoptee problems arising from neglect/abuse/trauma in early life but not much about this particular issue, AFAIK, which is equally hard to recover from if expert help isn't provided at the right time. I agree with a PP, I suspect that it is far more common than is acknowledged.
I think more help is needed - to help potential adopters understand that adoption will not heal their losses in the long run and to help adoptees with their losses.
OP and @AnotherCrappyDay I think that what might turn things around also is to shift your expectations and approach the relationship with the DC in a different way. By meeting their needs including their need for love and connection in a different way you may then develop different feelings for them and they for you - and leaving you guilt free to get help for you when you have your own time. When you are with the DC you need to be emotionally present and available for them - I think that it is also normal for parents to drift into their own worlds at times, but once aware that you are doing it, it is easier to pull back - and for you to deal with your own grief and feelings when you are on your own.
If you were able to write down exactly what has happened with you from the start, and how you have felt every step and send it to every decision making or influencing body re adoption, eg the CEO Adoption UK, head of Children's Services, psychologists who work with adult and child adoptees, it may well be one small step towards things changing, and also may help you to write it all down too, if you haven't already done that.
I think you are right that it would have been very different if you had had birth children, to be fair, OP. I think adoption is great but it isn't the same as giving birth. But it might be that you are having problems attaching/bonding/parenting to do with your own upbringing and that that is feeding into your other unresolved feelings of loss.
NB it is normal for DC to start to be more influenced by peers at this age and you can find guidance for parents on this subject on parenting sites.