A classic long sleeve to pair with anything. 100% waffle cotton.
For reference Sonny in first few pics (petite 8) wearing 6-8 + Valli (petite 6) is wearing 1-2 (coz that's the only size I had at the time!) .
Would recommend just going your usual size but please check measurements below if unsure.
MEASUREMENTS (taken while lying flat)
6-12 months - chest 28cm length 35cm
1-2 years - chest 31cm length 40cm
2-4 years - chest 34cm length 45cm
4-6 years - chest 37cm length 50cm
6-8 years - chest 40cm length 54cm
8-10 years - chest 43cm length 60cm