Short of cash?
Miscellaneous: There's a cash dispenser in the Willem Alexander pavilion. Just in case you buy too many souvenirs! It's located on the right after you enter the pavilion through the southwest entrance (where the restaurant is).
A day of photography
Luggage and bags: You'll be walking so take care not to have to carry too much.
Clothing/Shoes/Weather Gear: Sensible shoes for a longish walk. The paths are well laid out and if you wish high-heel proof but you'll like to be comfortable I'm sure. Bring an umbrella if it threatens to rain the day of your visit. You can buy them at the park but you'll overpay.
Toiletries and Medical Supplies: Bring whatever you need for the day, you won't be staying the night in the park anyway. For babies there are nappy changing facilities on various locations in the park (toilets).
Photo Equipment: Bring enough memory cards and well charged batteries because you'll want to catch those brilliant colours!
Miscellaneous: Bring a picnic. You might want to enjoy lunch sitting in the sun in the park on a bench or grassy fields (where it is allowed, of course).
And if not, bring cash money for the icecream stalls, the hot dog stalls, the restaurants.