In zone 9 why not just plant them outside? They need more light and they only good way to do that is outside planted where they can fruit.

Fruitnut is right. I am in zone 9 also and I direct sow the watermelons outside on March 1.

It was a spontanous thing. We were in the garden center and my kids wanted to plant seeds in one of those self watering seed planting kits. We plan to transplant them into Earthboxes. So I am guessing the grow light isn't enough? Are these going to the trash or are they saveable? Is it too late to direct sow into an Earthbox?

Trash.... Just direct sow the seed outside in the earthbox. Watermelons need a lot of feed. I don't think it is too late.

Or, you might save the kits and lights to start tomato seedlings next winter. This would allow you to get varieties not available commercially. The grow light should be just a few inches above the plants, and they will need air circulation and some attention to temperature. Check on the web for instructions. You can also get a little jump on the season next year with other warm-weather crops by putting down clear or black plastic to warm the soil while planting seeds indoors. Then plant the young seedlings through the plastic mulch.