I, personally, would feel OK with it.
He sounds very responsible and as long as you trust him to stay inside and not answer the door, I think it sounds like a good compromise. DS is 6.5 and I sometimes let him stay inside and play while I am outside with the neighbors and dd. I'm so close and he is extremely trustworthy.
I am also struggling with the different needs/wants of 2 children! I think it more challenging when their age differences are further apart (3.5 years, in our case).
He sounds very responsible and as long as you trust him to stay inside and not answer the door, I think it sounds like a good compromise. DS is 6.5 and I sometimes let him stay inside and play while I am outside with the neighbors and dd. I'm so close and he is extremely trustworthy.
I am also struggling with the different needs/wants of 2 children! I think it more challenging when their age differences are further apart (3.5 years, in our case).