To add to the ideas suggested already,

How about a brief introduction to the concept of flags?

Showing the American flag (and accompanying brief
discussion, using the globe of this being a world,
and "where in the world do we live..." our country
is America and information about what all the
celebration is about- America's birthday.)

the concept of flags can be introduced. Flags have
different designs and colors. Why? Sometimes these
designs and colors been chosen for some special
purpose, have some meaning.

Activity- design your own flag. What would you like to
have on it?

Can use an old sheet, and fabric crayons, or markers and
paper,any art materials you can come up with which
will allow the children to be creative about making
their own flag. After the flags are made, you will
want to have a parade, no doubt, with music.

And it will be very interesting to ask, "can you tell me
about your flag" and see what the children are thinking
about, how they have chosen to decorate their flag, and
why.