Yes, get it done. Both of my girls have gotten one since they were old enough to receive them. They are aged 8 and 4 1/2 as well. No, they cannot get the flu from this shot. A lot of people seem to get the flu from the shot, but it is usually because they already have it in their system right before they get it and it seems to flare up at the just right moment,thus people blame the shot when they get sick.
I am not a health professional, but have asked my pediatrician the same questions you asked and he reassured me of same concerns you have.
My kids did not get sick after getting a flu shot.
Hope this helps, other people may have had different experiences that I have had though. Several of my other friends had their kids done as soon as they were old enough and their kids did not get sick either.