While i can see your ideas on flexibility op i fear you are going to leave yourself open to further abuse by doing this . He will not see this as you being decent , he will assume that sometimes he can bully you and sometimes he cant , in a few years you will be having the same old battle .I think you need very clear boundrys here , ie , if its his weekend he takes responsibility or he takes the consequences which will probably be kids who are very annoyed with him .
I think i would be having an age apropriate chat with the dcs about who is responsible for what and who is in what role , and that the current situation cannot continue . If there is a party on dads weekend and dads not prepared to take them i would suggest that they have 2 options , they can either miss going to dads or miss the party . If that means they start to drop weekends he will soon start to change his attitude , he simply cannot have it all ways .
These idiots are like bratty spoilt toddlers in adults bodys . They cant stand not to be noticed or to not have their ridiculous demands met . Its took over 2 years to train mine , and a big part of that was stopping sheliding the kids from it , and making sure he was responsible for the consequences .Initially if he was late and the dcs were getting upset i would make excuses for him , hes probably stuck in traffic ect . If he was a bit unkind to them i would say he probably didnt mean it ect . Eventually i realised i was feeding the kids the same excuses i had fed myself during our marriage .
I started to say that yes , dads late again and its not ok , and that actually that was very rude for dad to say that . Instead of arriving here 2 hours late to a happy kid who was thrilled to see him , he got an upset kid who asked him why he was always late and why didnt he ring . Of course he blamed me , he claimed it was me that had upset them by daring to tell them the truth , but i didnt give a shit , i wasnt going to keep covering up for him and making myself look bad in the process . I was sick of being the bad guy .
A positive outcome in this is that my kids , who are a similar age to yours have excellant boundrys . If he has been thoughtless or unkind they will calmly say so stating they dont like it and that they would like an apology . This terrifys him . He is emotionally stunted and is frightened by the prospect of having any real sort of conversation so this alone keeps him in check nowadays .