Childcare - who's responsibility?

[Jemimapyjamas]

In brief: my cousin and his girlfriend have a child starting the same school as mine (they started on Tuesday.) His parents often look after him when they are working (she works part time, he is full time) but they on a month away that has been planned for some time (almost 12 months.) I am a SAHM but am now, as of this week, doing more freelance work. Previously, my work fitted around DS who did two days a week at nursery.
I was asked at 4pm on Wednesday if I would have their son for Thursday and Friday this week, in a very 'while you're picking your DS up...' kind of way rather than that they had been caught short, sent at almost 4pm the day before. I explained I was working but could help out on Thursday but not the rest of the time. I did that, and it was fine but was also then asked if i could mind him Monday too. I said no as I was working. Today I got a text at 2pm asking me to pick their chid anyway as one of them was running late. My DH said (the text and then call was while I was having a Skype meeting) after that we can't be back up and pointed out they knew we / I / he (we both work from home) was busy. Also, its not like this is unexpected (as if it was, I would be happy to help.)
They are not, it appears, not speaking to us.
AIBU...?

You never know when you may need them to return the favour.

It is good to have people who can help you in an emergency (we have friends who we help out and they help us - either last minute emergency or preplanned but its no more than once a month). But these people sound like they cant make their way out of a paper bag and are also unlikely to reciprocate.
There's no need to be confrontational just say you're working. For me its weird you can look after your own child whilst working - this would be near impossible with my work and children (I also work from home).
Just say sorry, cant help you I'm working ;-). Good luck!

Thanks again for replies. I can vaguely work around one child, and while sometimes two can be easier as they entertain each other, it's not necessarily the case. I also have deadlines that need to be met in the morning so I can work later if I need to. I think it's the assumption, despite what i have said, that I will do it regardless that grates. Especially as they have laid low when we might have needed help (like when we moved house, we never heard from them at all.) I said i can help in an emergency but this isn't...

YANBU. You are at work. They need to make their own arrangements. End of.

LaLaRoo I think it's the presumption and also that they have had so much notice but not bothered to organise something else, just assumed. (I have looked after him a lot, and their dog even more, previously.)

I can see where you're coming from, but personally I'd just pick their kid up as well and plonked him in front of the tellybox. You never know when you might need the same in return.

...you never received the text.