@beAsensible1 OP has not said it is until April. OP put in a post - it is long term, April and beyond.
It's not just up to the OP's DH though is it? It affects the whole family, it's not like they are even the same ages or thereabouts, all those ages have different routines after school. How do you know, it's been thought thorough, he might have been put on the spot, agreed and now doesn't know how to backtrack. It's not that straightforward.
Life with primary school children can be quite unpredictable - what if one needs a doctor's appointment, dentists, playdates happen a lot at that age, parent's evenings, what if one of the children is ill - are they going to want other DC there. You can't say they will know appointments and activities, things change all the time and quite often activities change from time to time.
I don't imagine they have after school playdates that often, and if they do pretty sure one more won't hurt - seriously, can't believe you say this. IME lots of playdates happen at primary school age and then 5 DC after school.
I get why OP is annoyed but really if he's willing to take it and help out someone then its on him.
As someone posted upthread the level of insularity and individualism has to be let go. build community with people! - It's not just on him though is it, OP has medical condition and this will invariably impact on their own routine. Yes, it's nice to help out I agree but this is another commitment altogether.
He is looking after 2 children for a 2 hours! its not rocket science or life threatening. give them fruit and crisps, get them on with some homework and a bit of telly. He should ask for a little bit of money towards after school snacks etc. - Of course it's not rocket science, who is saying it is but it will be completely chaotic and anyone that thinks it won't is naive. If the neighbours want to move, they need to put proper childcare arrangements in place.
I just honestly think there are worst things in the world and its not that serious and we are infantilising OP husband as if he isn't aware of the burden he's taken on. - Well, yes again, of course, there are worst things going on in the world but that isn't the point. The OP can see this is going to have a significant impact on her afternoon/evening and is rightly concerned. We all have to have some space, routine, downtime so we can run our own lives and look after ourselves. If this was a couple of days a week, possibly okay but IMO it's far too much to expect someone to commit to.