I can't tell you what they look like, but if you've sown in open ground, I understand the problem. First, remember the initial two 'leaves' aren't true leaves - when the second pair arrives, these are true leaves, and should take on the form of the plant you've sown, albeit very small. It's always best to sow a few seeds in a pot as well so you can recognise them and know what to look for in the ground.

I think I found a picture, though the seedling looks young:

http://farm4.static.flickr.com/3064/2352674629_e953548369.jpg

I suspect the leaves become more thistle-like as it gets older.

I often use this site for identification of seedlings and sometimes propagation ideas.

http://theseedsite.co.uk/index.html

but I found meconopsis seedlings on this page. I hope this page works, as it is a bit of a confusing website to find what you want.

http://theseedsite.co.uk/db14.html

have fun

Thank you for your replies. I put them in a propagator outside like it said on the packet, but I have a feeling some previously-used compost might've got in there, hence not being able to tell whether the seedlings are weeds or not. They don't look anything like the image you found me Tugbrethil so I don't have much hope! Nothing else is in there as yet so I'll just wait longer. I'll then try again, knowing what I know now!