I went to a successful one a few weeks ago and they had a children's entertainer, the activities could easily be done by yourselves and suited all age ranges. Face painting Pass the Parcel Bouncy castle lots of 'making' eg. potato face, glitter, glue, colours etc All kids love balloons. I know a local 'balloon maker' near us who charges about £25 ph music
I would keep it simple, have bubbles, baby toys etc out for the little ones. Maybe a table set up with play dough or similar on. As last post says have a few games, preferably ones that they can all play (the big ones can help the little ones). You didn't say what your budget is. That wouuld help tp know what to suggets.