Another sub-panel question
According to Table 250.122 of the National Electrical Code, a 10 AWG copper conductor is fine as an equipment grounding conductor for circuits with up to 60 ampere protection.

National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 250 Grounding and Bonding
250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment. Where a cable tray, a raceway, or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or (B)(4). Equipment grounding conductors shall be permitted to be sectioned within a multiconductor cable, provided the combined circular mil area complies with Table 250.122.