I don't know if this would be retaliatory rudeness or not, but I would have postponed giving my baskets until the full exchange could take place. I would have phrased it along the lines of "It would be more fun for the kids to all have baskets together, let's reschedule for a time when you guys are ready."
I actually like this idea, and I don't think it seems like retaliatory rudeness. I'm curious to hear other responses.
IME, unless all parties had their gifts available to give out, then that's exactly what would happen - it would be delayed until everyone had their thing to exchange.
I can't imagine going along to a reciprocal gift giving event, and an established one at that, and receiving but not giving. Especially if there are children involved. I'd feel very heartless if my kids received the present from the other side, but we didn't give one in return.
In the OP's case it sounds as though the aunt and uncle never even organised a basket for the OP's kids (or gave it to their own kids), because they weren't getting together over Easter as per usual.
I think if someone is able to meet up at a later date to receive a gift for their kids (without their kids being present), then they can also come along with the reciprocal thing to exchange. No, no one is entitled to a present, but in something like this it just seems rather meanspirited.
I wouldn't make any fuss about this year to my in laws, but next year I wouldn't hand anything over unless it's actually at a gathering and all the kids are getting something. If there's no family gathering, I'd just give the gifts to my own kids (or eat them myself), and not worry about the exchange in the future.