Your attitude is bizarre and quite possibly rather nasty.
Maybe a joint party has been planned because this was the only way any of the mums could afford to do a party for any of the girls ? Maybe they too are facing redundancies or money worries (like many of us).
Even if that's not the case, and money was "no object", a joint party isn't a question of not making an "effort" but a practical solution to all sorts of practicalities like birthday date clashes, parents who work shifts and who only get 1 weekend off every so often, or parents whose other commitments mean it's a logistical nightmare ferrying children here there and everywhere on several consecutive weekends. But whatever the reason, surely the only thing any invited guest (or their parent) should be concerned about is the fact it's bloody nice to be invited to parties at all ........ I've seen several threads on MN from distraught mums whose children are rarely or even never invited to celebrations ..... maybe 'cos they're not part of a clique, maybe 'cos their child doesn't fit in, or has SN ...... but they feel dreadful on behalf of their kids who have to listen to everyone else excitedly talking about said party.
Most mums like getting party invites for their kids because it usually shows their kids are likeable and popular, and the vast majority of parents who send out invites do so because they want the company of their child's friends ..... not because they have their eye on a stack of presents as some sort of pay back for the invite.
If you would normally buy each of these little girls a gift because they are your daughter's friends then what's the difference if you buy 3 at once or 3 separated by a week or so. If money is tight after Xmas, or at any time, then you cut back accordingly ...... aside from recycling unwanted gifts of your own, go to Poundland or somewhere similar and pick up felt tips & colouring books. Done. They are only 4 FFS, they are not going to be totting up how much you've spent. Unlike you, who has totted up how much their parents have spent (or not) and become affronted by it.
One of my daughter's friends who's born in the summer luckily had a "party in the park" which is what it sounds like. Mum bought drink and cake and the kids played on the swings and had a great time. Did she do it to save money ? .... I've no idea, I just thought it was a damned good idea as there was plenty of space, plenty of entertainment, plenty of fun to be had and no mess. Working on your rationale, presumably this woman had made no effort whatsoever and her child didn't deserve a present.