Hope you aren't getting too bored of this yet. I swear I'll move on to something else soon ;)
Then because the white bag was quite thin, and the tshirt that it was going to be stuck to was a dark colour (I guess you wouldn't have to do this with a white shirt), I cut out a slightly larger heart in a thicker yellow plastic to go under it. I ironed those two pieces on at the same time.
Then, I cut out a larger clear plastic heart shape from a little freezer bag. I think thinner sandwich bags would be fine for this, probably even better, because the thickness of the three layers of plastic did make the finished decal about as thick as a regular iron on decal.
And here is a pic of the parchment paper after it had been used to iron on the top clear plastic layer. You can see that some of the ink has actually melted through into the clear plastic, so I'm betting that's pretty well set and wash proof.
This one hasn't been through the wash yet, so I can't vouch for it's durability, but it seems that there should be no way of the marker pen being washed out from under that final layer of clear plastic.
Obviously grown ups could have loooooads of fun themselves drawing and fusing their own artwork this way, but I thought you'd like to see something that the kids could have a go at themselves.