Strange how different the vote is from the comments.
I agree with most of the comments.
In your OP, you've made all sorts of statements to get us to agree with you, but most of them don't stand up.
For example, they aren't "unfamiliar children" if he walks them to school each day - you might not know them, but her does, and your dc do.
I feel it’s a lot to look after 4 children (who don’t know each other)
Not really. They aren't babies or toddlers. They will just sit there and watch TV or build some lego or kick a ball about outside, or help your dh in the kitchen, read, or even do a bit of homework.
I honestly don’t know what they will do - to entertain themselves. we don’t have an x box or anything as my children are younger (3 and 8).
See above
I don’t like the thought of unfamiliar children with my own children being left alone.
But, they aren't alone. They are with your dh, and their friends that they walk to school with each day.
Your dh sounds like a nice chap, who has kindly offered to help out people he has got to know, when you are out at work.
If you really need to lie down when you get in, you still can - I am sure the dc will understand if you want to lie on your own settee, or you have the option of lying on your bed if you prefer a bit of privacy.
Of course, they might not still be there when you get in.