Another IL complaint

Ugh..they're driving me nuts again. DH and I mow our respective parent's yards during the summer to make some extra $ (esp now that we have to buy another vehicle). The arrangement so far (all summer) as been that DH picks DD up from my sister/mom's house (where she stays all day) and takes her to visit his parents while he mows. This is the only time the ILs see her since they flaked on watching her once a week, and they frequently complain about how they "never get to see her". Well, we're supposed to mow today. It's 2:45 (DH gets over there around 4:30) and my MIL has just told DH that they have contractors coming over tonight and don't want DD at the house...they have known all day that he was going over there and they wait till now to let us know. My mom works nights and can't watch DD on the evenings she works (which she does tonight). My sister watches her all day and enjoys having a break in the evenings (I don't blame her..I enjoy my time away from work too, even if her work is cuter than mine). MIL just expects my family to drop whatever plans they have last minute an watch DD because it didn't work into her plans. I wouldn't care if she'd said something yesterday or even this morning- I could have called my Dad and seen if he could watch her (it takes several phone calls to get ahold of him as he doesn't carry a cell and doesn't usually listen for the house phone and now I don't have time to fool with all that). My sister has begrudgingly agreed to watch her, thankfully, but she's not happy about having to give up her off time to do so. My only other option is to not mow and miss out on the $50 I get paid to mow, but we really need the money since we're having to buy DH a new (to him anyway) car. If the situation was reversed and my parents flaked out and expected his parents to cover, they (the ILs) would be livid. I just don't get why they can't be bothered to fit time with their grand-daughter into their schedules, and then have the gall to b**ch about never seeing her...hello..it's your own faults! To top it all off,  DH always makes excuses for them or tries to apologize for them...they're not sorry. He won't stand up to them about it though, because "he's the child and they're the parents" and if I say anything, my MIL will turn it into some giant war with DH and DD stuck in the middle (she's done it before, pre DD). So here I am, stuck scrambling to cover for them at the last moment, and nobody but me seems to give a dang. !@#!@!@!@#.