Waimea Valley botanical garden, North Shore

Waimea Valley botanical garden is a paved walk to a waterfall. To the waterfall is one mile, and then you return the same way you came. The path is wide and paved, and very busy with tourists. Along the way you see gorgeous tropical flowers and incredibly old trees. The Albizia Saman Monkeypod tree near the cashier booth is truly awe-inspiring. A wide muddy stream with little boulders follows the path, and in some spots the path is high high above the stream, giving the walk a nice feel. As a tourist attraction, Waimea Valley also offers demonstrations of Hawaiian crafts, items from nature to touch, and items with historical/cultural significance on display. There weren't any actual activities for kids on the day we came though, such as lei making, though there was a Hawaiian tutu (grandma) teaching kids how to play a Hawaiian checkers game. At the end of the walk, you can swim under the waterfall, though it didn't look enticing to me, and there is a risk of leptospirosis. All swimmers must wear a lifevest, which is provided.

This is the place where the TV show "Lost" was filmed, when Kate and Sawyer swam playfully in the natural pool below Waimea Falls, before they located Kate's briefcase. 
Wear long pants because there could be mosquitoes, and avoid the lush side paths if you don't want to be bit. 
After your visit, check out the heiau, Hawaiian religious site of sacrifice, at the top of the parking lot.
Come to Waimea Valley for the Haleiwa Farmer's Market, on Thurs from 2-7pm (see address below). 
It's very expensive for a whole family to do this walk, so if you don't want to spend money, have an awesome hike for free at Pupukea Road Trail. Here you won't see a waterfall or flowers, but you will have the whole trail to yourself.
Across the street from Waimea Valley botanical garden is Waimea Bay, a super place for a summer swim or to watch the huge waves in the winter. This is one of the nicest beaches you'll ever visit! Play on the shore, letting the waves push you up the sandy slope and then pull you back down. Keep your eye out for freak large waves though, which can occur.
To introduce your toddler to tropical animals, get them this adorable touch and see book about rainforest animals in Hawaii, Rainforest Touch-N-See Hawaii. They will love it as my child did! And you can revisit your vacation memories every time you read it.

Directions

Waimea Valley botanical garden is located at 59-864 Kamehameha Hwy, Haleiwa, HI 96712, call (808) 638-7766. For your GPS, enter Waimea Valley Rd. Open daily 9-5. Closed Thanksgiving (fourth Thurs in Nov) and Dec 25. Cost is $16 for adults, $12 for seniors and students, and $8 for children aged 4-12. Local residents pay $12 for adults, $8 for seniors and students, and $6 for children aged 4-12 (bring your IDs). Individual annual pass is $50, or $100 for a family. 
Haleiwa Farmers Market, Thursdays 2-7pm, call (808) 388-9696.

Click on map for interactive view