Being a full-time carer is something that uses not only physical energy, but also our mental energy.
Just as we get tired physically, so we do mentally, and we take any opportunities outside the caring to slip into idle mode and re-charge.
Unfortunately, while we may appear attentive - and want to be so - when talking to someone other than the person we are caring for, we are often going through the motions, so to speak, and the content of conversations will not stick in our minds.
It is particularly the case at the end of the day that we are least likely to retain recollections of conversations held, because we are so tired.
I don't think there is much you can do other than explain to the boys just why......