I have just gone through this process my friend, I had a good dozen sprout from that kit. Once they grow a bit more, start splitting them out and put into their own pots. I'd advise something in the 4-5 inch range and let them grow.
I put mine in a small starter seed tray and kept them under a grow light, lost all but one due to too small of a vessel for the seedling.
Theres alot of people who will tell you that if you want true bonsai trees, start with bigger trees. I have found some joy in starting them from seed, but along the way I have purchased a couple dozen different trees.
Good luck,
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