Reading between the lines I think the OP's biggest issue, despite how she's phrased it, is with her DP, as opposed to the children themselves.
I really don't know what to think here and of course I find the thought of not feeding 2 out of 4 children cruel, but, despite getting together with a man who had children, I think there is a big difference between accepting they will be in and out of your life for xx nights a week or whatever, and maybe ... at a later date ... suddenly being expected, perhaps without any prior consultation, to become their full time carer during school holidays.
I don't know how the current arrangement has come about, whether or not it was discussed and agreed with ALL the adults involved but if the OP's DP has effectively given her no choice in the matter, then I too would be feeling very put upon and trapped. This is NOT "wrong", nor is it an indication you are an evil witch who needs to be burnt at the stake ..... after all, the OP may have made her choice to be with her DP based on a particular set of circumstances, only to find that he then moves the goalposts spectacularly after she's given up work, had children with him and so on. If his older kids had previously been with a childminder for example, the OP may have (not totally unreasonably) thought that would continue. Alternatively, had she known that it was likely she'd also be required to look after her skids if she had kids of her own with this man, she may have decided that it was more than she'd want to cope with (as is her right) - 4 kids are a handful that not everyone wants to take on.
I am only speculating at this point, because I don't know how all this has come about, but putting forward another possible point of view ............ (I can't imagine that anyone would go, fully aware into a new relationship, knowing that if they ever had children, they would also then be expected to look after 2 more, and complain about it after the event).
I do feel very strongly that the children are foremost the responsibility of their actual parents however, and it is up to them to ensure that they are safely and appropriately cared for in the best environment possible .... whether that be with a (willing) stepparent, or a childminder, or at a holiday club. It isn't right to simply assume that as the OP has 2, then another 2 won't make any difference ....... we all have our breaking points, and different tolerance levels.
However, for the moment, while they are there, then of course she should try her upmost to make them feel as comfortable as possible. I'm hoping that some of the stuff she's said has come out wrongly as a result of tiredness and frustration as opposed to a desire to be genuinely hurtful or cruel. Equally, while they're in her care though, they must abide by house rules and again, this is where her DP should be helping to ensure that they do. No-one, a stepparent or a childminder, should be held to ransom by children doing things their way all the time, particularly if that impacts upon the rest of the household.
My advice to the OP would be to thrash all this out with her DP because the current situation doesn't seem to be benefiting anyone except the 2 actual parents and somehow, that does seem unfair. If the OP is desperately unhappy, then whether anyone here thinks that's right or wrong, that unhappiness is going to be conveyed to ALL the children concerned one way or another .... and what's important (so no child suffers) is that the source of that unhappiness is addressed.
I also get the impression, again reading between the lines (though some of it is a bit confusing about who's working or not) that the OP's DP is insisting on "his" children being in "his" home when he's not there, which seems pretty pointless if there are occasions when they could be with their mother. If there's an element of point scoring going on between the Op's DP and his ex, with the kids caught in the middle, then again, I can understand why she'd feel resentful stuck in the middle of their battles.
You really, really need to have an honest talk with your DP and if he is "jumping" to his ex's demands, and this is to your detriment, then you need to put your foot down.