Many complicated topics in biology can be explained with a detailed understanding of DNA's structure.  There are four nucleic acids in DNA which all share the same backbone so I have made this model where each nucleic acid has a black peice of plastic, the backbone, and a brightly colored peice of plastic, the base. The first thing this model will teach is hydrogen bonding among complimentary bases.  Once you have inserted magnets (like this) the nucleic acids will snap to their pairs, adenine to thymine, guanine to cytosine.  The magnets are simulating an interaction caused by the hydrogen atoms having a positive charge and oxygen or nitrogen atoms having a negative charge.  

Once hydrogen bonding is understood, have the kids play around with making a chain of DNA back bone covalent bonds.  I recommend using the "backbone-black-immobile.stl".  I spent a lot of time learning how to make the "backbone-black-moreMobility.stl" but when I tested out how kids used the product most kids were much less excited by the higher difficulty backbone-black-moreMobility version.  With either backbone model, to make the chain, snap the peg part of the black peices into the socket part of the black peices.  Ask the kids to recall the way adenine bonded to thymine and guanine bonded to cytosine. To make the famous DNA double helix they are going to need to take their chain of DNAs and bond it to a complementary chain of DNAs.  The easiest way to make a double helix is to take a chain and a single DNA, find the single DNA's pair and hydrogen bond them.  Next, hydogen bond the next DNA in the chain and covalently bond the backbones of the two DNAs in the compementary chain. As your second chain grows in length the shape of a double helix should form.   Congratulations! Have the kids noticed that the two chains of the double helix point in oposite directions?

to simulate "unzipping the DNA" pull the two chains apart because the covalent bonds in each chain are much stronger than the hydrogen bonding the two chains can stay chains even if the helix becomes unstable.  It should be easy to get the two chains back into a helix as long as the DNA sequnces are still complementary.

Next maybe try the same thing with the backbone-black-moreMobility stl.  It has all the joints of flexibilty that a DNA molecule has