Dear

Our records indicate that two years ago you purchased a
bicycle for your son,  [name of child]  for his birthday
which falls on     [date]

If he is ready for a larger bicycle, you may wish to
surprise him on his upcoming birthday with one of the
beautiful models we presently have available in our shop
which would be suitable for a young man his age.

We do offer a modest trade-in allowance and, of course,
will be happy to deliver the bicycle to your home in order
to keep it "under wraps" until that special day.

Give it a thought and if you feel that   [name of son]
would be pleased, come in to visit us at your convenience
and I will show you our stock.