Both boats are fine
– Last Updated: Oct-06-12 1:26 PM EST –
for the intended use (i.e., paddling with your 6 year old ).
The Coleman brand kayak is in fact a Pelican APEX 130T. I believe (but am not completely sure) that the Pelican kayaks are not roto-molded from single mould (like most other plastic kayak), but are assembled from two separate pieces (a top and a bottom). While the paddling.net reviews for the APEX 130T are uniformly positive, a frequent criticism of Pelican sit-on-tops is that they leak from the seam between the two halves. This would be something to check before purchasing, (for example by filling the hull with water)
From your message it is unclear to me whether one of the kayaks is an "Ocean Kayak" brand "Malibu Two" boat, or some kind of double from the "Malibu-Kayak" brand. In either case the boat will be rotomoulded and therefore less prone to this problem. Even roto-moulded kayaks can leak however; the scupper holes are a common weak spots.
The OK Malibu II is probably the most common/popular sit-on-top double and is a tried and tested design for your purposes. Willowleaf probably knows more than I about the price situation.
Good luck with your purchase. And don't worry too much, once you have the boat, you will have fun!