Ideally try to use a felt or leather plectrum, on the ukulele. Although I have known many uku players who use a plastic plectrum (the same as the guitar) the nylon strings on a ukulele are soft and can break if you use a heavier guage pick. If you find a felt or leather plectrum a bit cumbersome on a small ukulele. Try a narrow plastic one. A narrow guage is anywhere between 40mm to 60 mm.

£4.40 Ortega 4mm Hard Felt Picks, Pack of 3

Narrow guage plastic plectrum