The beach at Poipu Park is ideal for kids. There is a very shallow water area with no waves. It's also good for beginning snorkeling if your 5 yr old wants to try that. There are some hiking trails that start at cliffs at the beach that is in front of the Hyatt Hotel.

If your 9 yr old wants to try riding waves on a boogie board, the safest place for his age would be Kalapaki Beach at the Marriott Resort in Lihue. Kids love the Hawaiian style hot dogs at Puka Dogs hot dog stand in the shopping center in Poipu Beach.

This URL has lots of good advice on Kauai:
http://virtual.yosemite.cc.ca.us/ghayes/Kauai.htm

If the weather is clear, drive up Waimea Canyon for some marvelous views. There IS a hike to a waterfall in that Canyon, but it is NOT one I would call "easy."

You can see some nice waterfalls with hardly any hiking at all:
www.hawaiiweb.com/kauai/sites_to_see/waterfalls.htm

If the weather is a little choppy, definitely go to the Spouting Horn.
http://www.youtube.com/watch?v=57ltI-UQXiA

If your kids (or you!) are big fans of Indiana Jones, don't miss the view of the peak that Jones saw in his first adventure.
http://www.indygear.com/igDWR-peak.html

Puka Doggggggs

Rent bikes in the Kapaa area and ride the bike path up past Kealia Beach over look.

Mini-golf up in Kilauea.
Kamalani Playground near Lydgate.

Kal, how long a ride is that bike ride (miles-wise)?

About 1 six pack.

Seriously: http://www.kauaipath.org/content/ke-ala-hele-makalae-project

We rented our bikes at the Islander (Behind the Coconut Mktplace) and rode on the hiway up to some of the side streets we knew that would take us the Kapaa beach park and basically the beginning of the trail.

We figured it was a 6-8 mile RT. Beautiful ride. Not too hard. Some nice photo op places along the way.

Perfect, thx

I forgot about the train ride thru the Kilohana Plantation.
We are going to try to do it this trip...along with a quick stop at their rum distillery.

There is also a Kauai Childern's Discovery Museum in the Safeway Center in Kapaa.

The Kealia Bike Path is easy, rides right alongside the beach but don't swim there unless you're a strong swimmer. Good to watch surfers. Bike rentals have tandem, 2w and 4w bikes.

Lydgate Beach Park has two areas for swimming and snorkeling. Both areas are protected by seawalls. One of the pools are quite shallow. Good for little ones who can't swim. The park has a fun playground with a volcano themed slide. Lifeguard.

Kalapaki Beach is fun. Sometimes the shorebreak is rough. No lifeguard.

Poipu Beach is pretty safe. Lifeguard.

Kilohana Luau.
Wailua River Boat Ride. Goes up the Wailua River to a fern grotto. Short informative program.
Shave Ice at Jojo's in Waimea.

Must try the local foods. Best kalua pork @ farmer's market @ kauai community college, saturdays.

Have fun.

We did the Kilohana Train ride last week and really enjoyed it.
We even ran into a Fodor's Politcal Discussion.

We also did the luau at Kilohana and it was very nice and the story they told was very respectful of the culture and at no time did they pull someone out of the crowd and make them do the hula.

Nice to see a lot of young locals embracing their culture like that.

We'd go back.

We also went to Kauai in August a couple of years ago when my kids were the same age as yours. We also stayed near Poipo beach which is a great beach for kids. You should also check out the northern side of the island. We really liked Hanalei bay and Annini beach, in August the water is very calm on that side making it very safe to swim. You can rent paddle boards at Hanalei bay which looked like a lot of fun but we didn't try it.

We liked Queen's bath which is a natural pool next to the ocean, it's a little hike down a kind of steep path but my kids did fine with it.

My 9 yo wanted to do ziplining too and I looked into it, there are some that allow children to do it but I don't remember now which ones. It was kind of expensive so we decided not to do it and I also wanted only do things that all of us could do.

We did a kayak tour on the Wailua river that included a hike to a waterfall. The kayaking was hard for me though b/c I was in the kayak with my 5 yr old that couldn't really paddle and I had to do all the work but it was still a lot of fun.

We also rented bikes in Kapaa, the trail is very easy. The 5 yo couldn't ride a bike on her own so we got a tandem bike.

You will have a great time, we did!