Haven't been to the Statue of Liberty in Many years, but I took 2 classes on class trips there. It is great to go to the crown, but IMHO, time is better spent at Ellis Island. If you have to choose between crown or Ellis, take the time to walk through Ellis & look at the exhibits.
You can see the beauty of the statue of liberty from the ground, and get a feel for how tall she is by going to the pedestal. If you can manage climbing to the crown & going to Ellis Island, by all means, make a day of it.
You may want to pack a lunch & have a picnic somewhere on the grounds outside.
Also, if you are looking to make Ellis island more exciting/educational for your kids, they used to have a scavenger hunt form. I called & they mailed me one which I copied & gave to my class of kids to make them focus as they walked through the exhibits. Again, this was many years ago & I am not sure if they still have this available (unfortunately I never saved a copy of mine) but it may be worth a phone call if you are interested.
Have fun!