Why are the girls so worried that the boys have witnessed the game?

A
reader can find this information beginning in
chapter 12. The boys in question are Ken and Toby,
and they are not exactly
known for being the nicest boys. They are school jokers, who do not
take a
lot of things very seriously. In fact, they are prone to giving people dumb
nicknames,
and the girls are not keen on having that kind of influence in
their game. They have built it
into something really great by the middle of
the book.

When Ken and Toby
discover the game in chapter
12, the girls are at first worried that Ken and Toby are going to
tell on
them. They are also afraid that they might not get to continue playing the game and
that
a lot of other kids will make fun of them. That is a real possibility,
with Ken and Toby leading
the charge. Elizabeth eventually comes up with a
last-ditch solution: to invite the two boys to
join the game. Ken thinks it's
stupid, but Toby is intrigued. The boys eventually agree that if
they don't
tell anyone else about the game, then they are allowed to play. April is
still
devastated by the turn of events, and she doesn't believe the game will
ever be the same with
Ken and Toby present.

We just cant play the Egypt Game
with thosethoseboys
there.

Fortunately, Ken and Toby turn

out to be really solid additions to the Egypt Game.