We earn £40k between us (before tax) and have four kids. We go on holidays, have a large car (VW Touran), have nice stuff (although we're not really 'stuff' people, not bothered about big TV's, fancy clothes or brand names particularly).
I think it's perfectly doable on an average wage. We have a large house (5 bed, 4 reception rooms) so plenty of room for everyone. My children are always very clean and tidy, well-fed, are doing excellently at school, play instruments, go to clubs, we go on family days out every weekend (forest, park, cafes etc)
I was one of eight and we had all this growing up (although my parents were considerably less well-off than we are now). Out of eight of us, six got straight As at a-level. We all play a musical instrument to grade eight level.
I think it's very much a case of what type of parent you are, not how many kids you have.
Expecting an older sibling to look after your children or do an excessive amount of housework to facilitate your lifestyle is shit parenting however many kids you have.