Posting 2 almost identical photos won't improve chances of ID. The leaves are still quite ambiguous.
You originally mentioned seed was small. How small is 'too small to see'? I can't think of very may trees that have really small seeds and these leaves do not look like any of the ones I can think of.
Any chance there could have been some stray seeds in the soil - compost or some soil component stored outside at any stage?
My best guess from want I can see is rosacea family - hawthorn or apple but given such young leaves could be way off and both have larger seeds. Sill vaguely possible for Chinese elm but as mentioned seeds are usually flat papery and around 5 mm in diameter.
Given the pack was grow your own bonsai that generally limits the choices to just a few species. Definitely not conifer so we can rule out pine and juniper. Chinese elm will be a strong possibility. Zelkova might also be possible from a bonsai seed packet. Does anyone else know what species these packs have?
They will do better outside as soon as frost has finished. Sunlight is good for most plants but as we do not know the species you will need to be conservative. Some are more cold tolerant and others will die with just a little cold. Some love full sun but others prefer filtered light. The species included in bonsai seed packs tend to be hardier types so I would be happy to assume these would be quite cold hardy.
The pots seem to be relatively large so they could be left in these for the whole summer. It is also possible to transplant and root prune at this stage. Just different possibilities. One way is not always better than another, especially right at the start.
You can start to feed little seedlings
Thanks for the reply,
Quite small is between 1mm - 3mm, most of the them were 1mm and looked round, although hard to tell at that size and black in colour, In fact apart from a few more distinct looking seeds it just looked like a small pinch of ground pepper, but a few were almost flat and disk like with what seemed to be a central disk core of a brown colour with an outer paper like covering. These were the bigger seeds at 3mm but no bigger than that.
The soil is commercially brought potting soil in a sealed bag I brought for the purpose, of course there could have been something already in the soil mix when bagged.
I have searched the internet for any idea of the contents of the pack and found nothing but plenty of adverts for selling them. So I have little to go on. I planted all the seeds in this propagation tray and three shoots appeared, the other two died off after producing the first set of leaves, but this one seems to be growing.
The tray has a clear plastic cover and so in a week,or two when frosts are less likely I will put it outside and see if it continues to grow, pretty sure indoor is not where it wants to be.
I will keep going until we get to a point that an ID is possible.