Glyphosate (Roundup) will kill all that. The trees, if they are over a couple of feet will need to be cut, so cut them and paint glyphosate on the stump around the cambium layer (green inside bark) to kill down to the roots and dig them up after a few moths. I have about 3/4 acre that I spray once or twice a year (on the weeds, small trees) and keep it pretty clear.

Jon

For killing the unwanted trees. triclopyr (an active ingredient rather than a product name) is more effective than is Roundup (active ingredient glyphosate)

Many tree seedlings will die if simply cut and will not regrow, so most of the time no poison spray is needed.
If those tree seedlings have grown too large for a lawn mower to cut off a saw of some kind should do just as well, especially since that is what will need to be done if a plant poison is used and they die from that. They will still need to be cut.
I would mow the area first before spending any money on plant poisons.

Thank you for the suggestions. I do want to plant in that soil so will the Roundup make the soil unusable?

No, contact with soil deactivates Roundup, you can plant within a few days.

Itilton is right about Roundup being rendered inactive rapidly on soil contact. However, many premixed Round-up products are a mix of Glyphosate (the active ingredient in Round-up) and some other chemistry.

This is done because while Round-up is effective against most plants, it takes a week or so before you notice it working. Adding another chemical to the blend provides a faster visible response. Some of these chemicals have a short residual effect though, so consult the label of the product you use.

While glyphosate is fairly innocuous by itself all of the sprays sold have other things, usually listed as inert ingredients, that can combine with the glyphosate and produce something that is quite evil. Aside from the expense of the product, very pricey, there are other, more environmentally better methods of control, available.