You are me, except that it's my parents, not my in-laws.
The reason your ILs are stepping up the blame is probably because your kids' energy is ramping up since they last saw you, and therefore they think they have more stuff to gripe about.
Which means that in essence they were always the bitchy kind, they were just waiting for some ammo.
Honestly, limiting contact is the best answer, and also easy to apply since you don't see them often.
If they criticize you behind your back, ignore. DH should present a united front with you by repeating to them exactly what you two decided you would say when confronted with stuff like that. No explaining is necessary, it merely puts you on the defensive.
And if they say something directly to your face, look them straight in the eye and say clearly that you are dealing with it in your way. If this leads them to retort that your way is wrong, raise your eyebrows and remark calmly how rude they are.
Calling them out in an unruffled manner is the best option. It destabilizes them because you are exposing their rudeness yet it also projects your strength in that you do not show that you are perturbed by their attacks.
DH does this perfectly with my oh-so-annoying mother.
One last thing: consider their criticisms coolly in the privacy of your own home. Do they have a point? Could you be doing something differently? They don't have to know you're thinking about that