Yes, seedlings will push up through the mulch. Most seeds don't need to be planted very deeply. The rule of thumb is to cover them with an amount of soil equal to the size of the seed. If the seeds are very tiny, one just presses them lightly into the soil.

Lorna

Poppies need light to germinate. You can plant on top of the leaf mulch, and their roots will go right through the mulch. Planting this way helps the poppy seedling from having to compete with weed seedlings. If you have seeds that need to be covered, just plant on top of the mulch and cover lightly with promix, sand, or potting soil. The less competition with weed seedlings, the better. If you disturb the soil you'll have to deal with the weeds germinating.

OH that is an interesting idea. So just scatter some soil on top of the mulch? I don't quite get how that mulch will be incorporated into the soil. I feel like I should dig down a bit and mix it in, but I'm afraid to disturb stuff.

I use leaf mulch as a cover mulch to keep out weeds. In one season it will break down to help the soil, but if you need to incorporate it into the soil, you will have to dig it in, and then remulch to keep down the weeds. Then you can seed for your flowers the way I described above. This is the only way I ever sow seeds outdoors. I can't be bothered with competing weeds. Of course the mulch has to be fine(leaf, mushroom soil, triple ground bark), not the large wood bark mulch.

Here is a link that might be useful: My Garden Travels

Where do you get mushroom soil? I've always heard that is wonderful for your garden, but I haven't been able to find any.

So, the leaf mulch will break down after one season? That is good to know. I have TONS of it and it is free, which is certainly a plus.

Mushroom compost is sold as a bagged product in most places which sell any sort of compost. It has been sterilized, so it doesn't have the living organisms one expects compost to have. However, it provides very good food for worms and microorganisms. I combine it with other composts.
Lorna