Often text in books, on microfilm or fiche can be small. The center will probably have one, but it's better to be safe then sorry. Take a tablet or scratch paper- Useful for copying down reference numbers or notes. A small language dictionary if you are researching in a foreign country. If you're taking children, bring quiet toys and books.
Children have short attention spans and libraries can be boring.
Bring a snack. Ask the staff where it can be eaten. Don't assume anywhere. Crumbs and spilled drinks can ruin books, films and fiche. Usually there is a designated area.