At 6 they may be ready to have the party WITHOUT the parents and that will also cut out the siblings in SOME cases. I have had parents actually TELL ME "He can only come if his little brother comes too!" SO RUDE! Try saying SOMETHING about "ZAC has only invited a few special friends so that he can have quality time with each. We really hope DANNY can attend." or "We have a lot of games prepared for the group of 6 year olds we have invited... i hope DANNY can join us for the fun" (then you can always say it would noy be an appropriate party for younger OR older siblings) You could also say "Feel free to drop DANNY off so you can have some alone time and or do something special with his sibleings.
Its sucha a hard one! It happens to me EVERY year too!
29 Palms, CA