I think you have to begin with her.
Think about the 'moaning' - is it serious or a passing comment? It is very easy to 'moan' in passing about something you don't want to change.
Also, do you think the partner thinks this is what parents do? I have known quite a few people who assume that parents pay for holidays (and even houses!)- they accept and say thank you nicely, but don't think it out of the ordinary at all!
Once clear in your own minds, I think you have to begin with her, at a time when the children aren't around - so may need setting up carefully. Then say, without complaining, that you are reviewing your finances(this keeps it less personal and critical). You can say that you have no wish to interfere, but she must be clear how much money you are able / prepared to offer.
Say - we are prepared to offer, for example:
something like one holiday a year with us (chosen by us), and the kids' swimming lessons as a regular thing.
Say that you would not see her homeless (unless prepared to take her in!) and would maybe cover the kids' nursery fees for a few weeks in an emergency but this cannot be open-ended.
Ask her how she feels about that and how she could organise their lives now that she is clear what can be expected. Say that if she needs other support and help, can she ask, so that you can consider it?
It sounds as if you have a good relationship, and I'm sure she will be grateful to you. Doing it this way means you are not 'interfering' unless asked, but you are talking about what is your concern (ie: your money)
Good luck.