There is no dress code for Boma. You are welcome to wear shorts, t-shirts, and other comfortable attire. (There is a dress code for Jiko next door. You can read about this
HERE.)
It is absolutely fine to bring children of any age to any restaurant at WDW except Victoria & Albert. Your toddler is welcome at both Boma and Le Cellier. There are high chairs readily available at both restaurants. I personally took my twins to Boma when they were about 16 months old and had no problems whatsoever.
The Moms Panelists have just been having a big discussion about Phineas
and Ferb. Unfortunately, these characters are not currently in the
parks. However, the characters available to meet at WDW are constantly
changing and there is always a possibility that Phineas, Ferb, and
their friends may be added at some point in the future.
To make sure that you're not missing anything, I would go to the Guest Relations office at any of the theme parks;
these are noted on the park maps. Ask them to call the "character
hotline" for you. The character hotline will be able to tell them if
any particular character will be in the parks on a particular day, and
if so, where and when the character will be there.
Good luck as you plan your trip to Walt Disney World!