If your DP and his ex had stayed together, and either of them had decided to begin a university course, the kids would have been affected. It is no different now - your DP can still make choices which affect his children's standard of living.
Of course he should make sure that the kids will have their needs met, and on top of that, it would be advisable for him to make sure that their current activities are not too much changed. Keeping in mind that family incomes do change, and that is true in every kind of family.
But it is impossible to give a definitive answer on this, because it depends on what the kids' living standards are, frankly.
Parents - together or separate - must often make choices which affect the kids in ways that are both positive and negative at the same time. When a family relocates for a better paying job...when a parent changes career path altogether for health reasons....when a parent decides to stay home to be with kids instead of using childcare...and yes, when a parent enters retraining or education with an eye on a better future...it is a shame that a father thinking about increasing his earning potential and employment possibilities, not to mention his personal potential, is being lambasted for looking at his options.
My DH and I were, until recently, both full-time postgraduate students. We worked part-time, and we have three children living with us. I paid for my studies through loans, and my DH received funding through a competitive grant (awarded to him based on the quality of his research). I have since finished my PhD and now have a job in my field which is an 'early career' position - so not enormously well-paid, but which is more highly paid than the work I could get previously, and which should be a pathway to increasingly well-paid work.
My DH is finishing his own PhD: his funding ran out last year, so we have been struggling a bit more, but we have been able to support the kids. Our income over the last three years (how long we've been living together), then, has been a mix of things - part-time wages, now my full-time wages, small savings from our previous jobs, and a pension received for the DSC's support. As far as benefits, we only receive child tax credits and child benefit. I have heard some criticism here of fathers who choose to pursue an education when they have dependent children, and all I can say is that my DH has never failed his children by doing this. Quite the opposite - They've all benefited from the increased time with their dad. He's been able to work flexible hours, and has been there for his kids day in and day out. When both students, we were able to arrange child care and domestic responsibilities between us. Should he now quit his PhD and get a 'real job'? Could he have worked instead, and brought in more income? Possibly (although to be very clear - the funding for his degree was based on his work - that WAS his job for three years, and it paid as well as any other he could get!) - but that would not have been a wise long-term decision, and it wouldn't have resulted in a better life for the kids. Another games system? Maybe - but they don't need that.
My DSD is about to start uni, and the grants/sensitive loans available are very, very good. You and your DP should have a look. There are online calculators which will help you figure out what your situation might be. Then he will be able to make an informed decision about what the choice will mean for his whole family, now and in the future.