Generally acrylic sealants are non toxic, but always check what you buy as with anything else.
I think resin sealers are more durable but also more expensive. Polyurethane is usually better, because its very thin, while epoxy has a very thick consistency. Although some companies make epoxy specifically for that job. Check the label on the product for number of coats, but i'd guess 3 would do the job.
I haven't given much researched to Apoxie since it's not available here in Bulgaria but its labeled as non-toxic.
I am sure Milliput is safe as I asked them directly (they replied quickly), but should be washed after it cures.
Generally epoxy clays are safe, but keep in mind that's for the cured product. That stuff is also really strong, significantly more durable than polymer clay. For me painting (though I don't have artist grade thick paint) the clay is harder and that's where the polymer clay gets a bonus with all its colors. Btw it air dries and its waterproof
I can't recommend any products since I am from Bulgaria and i don't really know what your shops got laying around. Most famous brands are easy to find around the forums. Btw check small stores, hardware shops and so on. You might find a cheap alternative. But as i said previously in the message, always check the product your buying. If you feel concerned, stick to the mainstream stuff.
I hope this helps