Is one allowed to remove fish bones from fish on Shabbos in order that a katan should be able to eat it?
The Peri Megadim writes that when it is impossible to remove the ochel from the p’soles, removing the pesoles is also called derech achilah, and is therefore permitted.
Rav Moshe Feinstein writes that since a child cannot eat fish with small bones, based on this Peri Megadim one may remove the bones immediately before eating the fish.
Rav Moshe continues that since an adult is capable of removing the bones from his mouth, the bones may not be removed prior to eating. If, however the bones are too small to detect during chewing, an adult may also remove the bones from the fish, but only when the fish is ‘on the way to his mouth.’
Concerning the general issue of separating bones from fish, the Chazon Ish (54) writes that bones should not be removed from fish, yet the Biur Halachah (319:4) writes that one need not tell off somebody who is lenient (see also Shemiras Shabbos 3:11-12), because this can be considered derech achilah. The Kaf Hachaim (319:44) is lenient concerning separating small bones from fish.