We do school work over the summer, to keep my old dd (who has LDs) from regressing too much. Right now I have my younger dd doing the 4th grade EM workbook (what she's supposed to start in the fall) in hopes she can skip to 5th grade EM by the fall instead.

I find that giving them 'control' over the work helps. As others mentioned, asking when they'd like to do it. I also let my younger dd pick which pages she wants to do. She seems to like this more and fights less when it comes time to do it.