Can certainly appreciate frustrations with "too easy uke". Run into it all the time.
I had the fortune (or misfortune) to have spent a fair number of years fighting "for real" in many situations (competition and for real).
Also study hard style of karate where young bonehead bang each other up. (BTW Aikido works very well on these guys!!)
Back to the story...
We have many, many new students in our dojo. They do not know how to attack properly. I figure it takes quite a while to learn how to be a good uke.
I try to teach them by being a good uke for them. As Senior students and Yudansha, you have an obligation to work with them as best you can.
Sometimes I talk to them if they are receptive and I "feel" they are at the point of that being the right thing to do.
Sometimes I kick them or hit them if they need it. (lightly of course).
But most of all, I try and set a good example at all times by taking proper ukemi for them and others.
And to be quite frank, sometimes I ignore them and just get through it and move on.
But I try never to be mean spirited, condesending, or rude...that would be the worst thing you could do.
Sometimes I wait till after class to pull them aside and work with them, if they are receptive to doing that....but never, never do I lecture or soap box. I always say something like....your really doing well...let me show you something that works for me... and leave it at that.