I think what you have to do is mentally separate the SC from how you feel about their mother, otherwise it’s going to affect your relationship with them.
I would also say that what your partner pays in maintainence should be separate and different to the amount you spend on them while they are with you.
To raise children is expensive due to feeding them housing them and keeping them warm this in turn means paying for rent/mortgage, bills, food and clothes. What is paid in maintenance is usually only a drop in the ocean to the cost of all that, however NRPs also have to live so again they can’t give too much to this if it means they can’t afford to live themselves. I may also get crucified for saying this but RPs would also get help towards this in the form of benefits and if they’re working they earn it themselves.
I do hear what you are saying, I have been brought up to believe that you get out of life what you put in and it’s wrong to use kids to get benefits. From your original post it appears that your SC mother has done this? It can get annoying when you see people get help in the form of benefits especially if you have had to work for less then what they’re getting. However, that’s a different issue entirely and benefits for raising children are usually calculated with the best interests of the children in mind so they’re not living below the poverty line, rather than allowing the RP to have a free ride! Believe me they are having to sacrifice a lot on their part in order to raise these kids.
I myself have a Dp who has a child from a previous relationship and we also have a LO together. It can get really frustrating and there are many Ffs moments especially when you’re struggling yourselves and you have to pay out for extra bits that you may feel should be covered under the maintenance and when weekends are swapped and missed at a drop of a hat, you do just have to suck it up unfortunately and not allow it to change the way you treat your SC.
There will also be many posters who reply on this that are in the same shoes as your OH’s ex so I don’t think you will get much sympathy on here I’m afraid.
What I do think you should do is have your partner communicate with his ex about money. Kids are expensive and they will always need new clothes and school uniform and supplies so it’s best to be as open as possible to the whole expense of it all and that way your Dp and his ex can come to an arrangement.
About access and seeing them during the week, I would say again communication is needed, maybe they can set out in writing what they want in terms of this going forward? Yes there are always going to be exceptions! For example if the children are ill or if they have an event on at school or with a friend so it’s a case of being flexible.
As a fellow step parent, I would say, it’s best for your own peace of mind to allow your partner to arrange this with his ex himself, don’t get involved with it, otherwise it can make you feel like crap and hold resentment towards the ex. It’s hard I know because if you see someone you love being treated like crap you immediately want to fight for them! His ex should also not expect your wage to be counted to what is given to your SC, as you are not their parent therefore not legally responsible for them, however as I have said previously your partner may need to sort this out with his ex without you there?
Hope you get everything sorted xx