re: Fiber rich foods for toddlers/young children

+1 on the oatmeal. Figure out whether she likes it in muffin form, warm as porridge, crunchy as toasted oat granola, baked into a high fiber loaf with whole wheat, etc.

A soft fruit like cantaloupe or other melon will add fiber and water to her diet. Try making low sugar (or watered fruit juice) popsicles; she may not drink but she might see the ice pop as a treat.

Straws encourage more drinking than cups.

Finally, individually wrapped prunes might do the trick naturally, and they're sweet. She might like them.