First come, first served
.... and with one short and sweeping statement you've totally dismissed the OP's problem, as well as, in effect, her children ... who also happen to be her husband's younger children. I like to think, and I'm sure the OP does too, that he has a responsibility to ALL of his kids and not just the ones who were "here first".
This situation is NOT so much about the logistics of the school run - though I can appreciate this is a particular concern of the OP's while her kids are so small. Yeah, she may get fit, but that's besides the point. IMO, this is more about how the OP's husband has agreed to something which will potentially have a negative impact on his younger kids, as well as on his wife ..... and I'm assuming from what she has (and hasn't) written that this wasn't so much conditions imposed upon him by the court which he had no say in but something he chose to agree to (guessing at the behest of his ex during negotiations ?) regardless of the wider impact upon children who are just as much his as the older pair.
In a so-called "normal" together family, the adults decide together how they allocate their resources amongst their children so that as far as possible each child is treated fairly. Sure, not everyone has limitless time, money and personal transport so in reality kids often have to take turns when it's impossible to be in two places at once - or they may have to curtail certain activities and spend less time doing them than they'd ideally wish. Shock horror - sometimes kids get told they can't do something because overall, the family as a whole just can't manage it and it wouldn't be fair to prioritise that particular child.
I'm sure the OP realised she'd be "signing up" for some "inconvenience" as a SM - it stands to reason that if you have stepkids who live elsewhere that's always going to be a possibility. But she shouldn't bloody well have to "sacrifice" her children in the process. And it must hurt terribly to feel that that is what her husband's done - he's sacrificed his younger kids because unless their circumstances change radically in the next few years, their situation means OP and the younger kids will often be effectively stranded in a rural location while the wants of the older children are always met re: parties and activities. Similarly, because he's decided that their limited financial resources will go towards fulfilling EVERYTHING the older kids want to do that in turn means enabling the younger ones to do similar things will become even more difficult, if not impossible due to lack of funds. How on earth can that be fair when his younger kids are every bit as "innocent" as his older ones and should deserve exactly the same consideration ? For now, the younger children will be oblivious to the favouritism and the practical difficulties dumped on their mother is the immediate concern, but long term, and more seriously IMO, how will the younger ones feel when they see their older siblings getting to do all sorts of things which they can't - and that their father is facilitating that ?
I see you have (at least) 2 children Oscar so perhaps, given you subscribe to the "first come first served* theory you'd like to explain how, in practice, you run a family where the precious first born child is perpetually favoured and placed first to the detriment of the younger child(ren) ? How, you explain to the younger ones why they are always left out, and why they get the crumbs left over once the older one has had their pick ? How you bring some children up as "second best" without affecting their self esteem ? I'm sure the OP would love to know because this is what she's facing ........ or, does the "first come, first served" rubbish only apply to step families ? In other words, it is right and proper to treat all children in a "together" family as equals but it is somehow acceptable to treat "subsequent" children in a step family as less deserving of their father's time, attention and money ?
OP .... I really don't know what to suggest as I can fully appreciate how "stuck" you feel about this and the situation is of course compounded by lack of funds. I can also see why you feel so upset because this isn't just about logistics but also about feelings - both yours and those of your children. Seems very unfair that he's made this rigid commitment to the older kids when he already has limited time available to see any of his children because he also works EOW. In an ideal world, that time should be shared out equally and I almost wonder if he's so readily agreed to this because if he's away with the older kids he's also opting out of the (arguably) more tiresome care that much younger children need. He's made this decision now - and I really do feel it's incumbent upon HIM to find solutions to the problems his choice have caused. Has he explained how he's going to be spending much quality time with the younger ones ? - how he's going to reassure them that they're just as important to him as the older ones? Or how they can do activities etc when they're older without a car or money ? etc etc. He can't just dump this on you without any further comment as if the younger ones don't matter. HE needs to be sorting out fall back plans for the school run so it doesn't become a dreadful and almost impossible ordeal - especially in bad weather .... HE should be sorting out help from other relatives or friends. Is there anything HE spends money on, for himself (e.g. drinking, smoking, hobbies), which can be "sacrificed" (and that's where any bloody sacrifice should come in) to pay for another car, or taxi, or to go towards the cost of activities, so that the younger kids aren't placed at a disadvantage ?
And even if he does manage to sort something out to make the OP's life a bit easier, that still doesn't alter the fact he made a far reaching and irresponsible decision where he didn't take ALL his kids into consideration. Which is what any decent parent would do. I think he's been very selfish, or very stupid, or both. And I certainly don't think the OP deserves any criticism - not once has she demanded that her kids should be prioritised - she just wants equal consideration given to all of them whereas the parent of all is showing favouritism.