It depends on the context you work in.
I work for local government in children's services/education department, so, if anybody is going to go with family friendly policies then this is it.
I work both part time/flexi time during the week and term time only, meaning overall I work for half of a normal week. There are several other people at my office who have similar patterns.
However, I have my own projects/work load I am responsible and do not leave others to pick up from me when I leave at 2.30 (apart from one other member of staff who also works part time but is afternoons) Because of the nature of our work we are very very quiet during school holidays. Staff who work full time get to take holidays during term time when we are busy and their work does need covering, but in turn work during the holidays when its very quiet. Nobody needs to cover for me when I'm off as my work only concerns school terms.
If I have to have time off to look after sick child then I have to make that time up.
Additionally, as I work 50% of a normal working day, I only get paid 50% of a normal staff wage.
If I have some work which needs finishing urgently, because I have to leave at 2.30 for school run and cannot stay until 7.00 or whenever to finish it, then I have to take it home with me.
I do appreciate how lucky I am to have the opportunity to work somewhere which is so flexible and helpful to me as a working parent, but I would absolutely refute the idea that as a PT flexible worker I do not pull my weight.