I don't think it's acceptable for them to be the only kids not going
As they are not your kids, then really, you don't have the luxury of an opinion. Not your DCs, not your problem. You can get your judgy pants in a twist all you like - but it is just hot air; you have no influence.
Why can't you accept the way your DP chooses to parent with his ex - if she receives CM via the CSA, then he's under no obligation to pay more, and he is the one choosing to spend your family finances on non-essentials for his DCs. I understand why that hacks you off, because its not a bottomless pit and her DCs get a lot more than yours, but you're angry with the wrong person - she is chancing her arm to get the most she possibly can out of her DCs Dad, but he is the one willingly giving it rather then saying no! She may well decide to pay herself if he lets the DCs miss out once or twice - all the while she knows he's going to fork out, she really is onto a winner if she doesn't pay, isn't she?
It's clear that you don't have any time for your DHs ex. She chooses unsuitable haircuts, prefers the DCs to be in childcare rather than a day out, she had other commitments at Xmas which limited her time with them and she plays the system financially. Not the kind if person you'd want to be friends with, really, is she?
But no matter how you feel about her, your DP will never feel the same way. You are not speaking for him when you condemn her.
She will always be the mother of his DCs - a woman he loved, treasured, planned a future together with - and while he may not agree with her choices now, there will always be respect for her, for things that they shared while they were together.
I don't believe that you are 'new' to step parenting - I think you have established your place in your DPs life over several years and he has willingly accepted it. Whether that is because of the love he has for you, a desire to give his DCs the best life he can, or in an attempt to get back at his ex, only he knows.
But it is not sustainable.
Your DSS, who is currently at school but oblivious to his haircut because he's so young - I give it a year at the outside before he does have an opinion of his own about his hair/appearance. Even in the smallest and most remote of primary schools, DCs are subject to peer pressure, face media imagery and aspire to look like their idols. What if he wants to look trendy, like his Mum encourages? Hes not operating in a vacuum, his Mum will encourage and support his desire to have a cool cut, designer shoes or latest shades - and this will all happen very soon! Are you and Dad going to insist? Are you going to enforce your rules and boundaries without flexibility and expect them to be complied with? Do you think your DSS Mum will force him to come to your home when he doesn't want to?