I am still working up how and what I am going to say to the two older ones
As someone who has learnt the hard way with my S17 I would recommend that you let them come to you and again don't say anything. You've set the pattern now. They will either keep it or they will tell you that they wish to do other things. That may not necessarily mean spending time with their mother. Just like initially we imagine the WAS and the OP doing all sorts of things together and having a fab time (when really it is far from that) it is the same with our children.
However, all of that being said you do seem to have 'nicer' children than mine so maybe you should go with your gut
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15