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My Celiac, Michael is a 7 year old, diagnosed since he was 3 so we've been
able to shape his habits.  However, some of this applies to all kids, no
matter what.  First of all, when we do eat out, we eat at restaurants where
we can get gf food.  For other fast food joints, I've made lists of what is
safe and what isn't.  So if his school class gets a treat from MacDonalds,
Michael tells his teacher what he is allowed.  If they have a pizza party,
Michael takes his own and has the teacher zap it in the microwave. (I make
small pizzas ahead of time and freeze them)  I've heard CD teens in our
group tell us that they used to take their gf pizza crusts with them to the
pizza parlours and have the pizza made and cooked there.  Of course, this
takes planning - no spontaneous pizza stops after the movies.  I send gf
cupcakes, muffins and cookies to school to be kept in the freezer.  When a
surprise is brought in my other parents the teacher just zaps Michael's
goodies in the microwave.  For special events at church, school or friends'
homes either I bake for all the kids so Michael isn't different, OR I call
ahead to see what is being served and send Michael the same thing but gf.
 Also, I keep lists of what junk foods are safe, i.e. potato chips,
chocolate bars, candy and allow him treats from time to time.  I think that
the key is this:  when at home you need to eat meals cooked from scratch -
these prepared dinners are full of preservatives and who knows what else.
 But, you have to allow for some 'junk' food and that takes planning.  Our
cardinal rule is that we keep Michael's life as normal as possible and try
not to let him feel too different from the other kids.  It seems to be
working so far and Michael rarely feels left out.  Of course, this takes
effort and lots of baking.  Hope this helps someone.