Found this.
NOTE: There is a strict dress code for visiting the Grand Palace. The Temple of the Emerald Buddha is Thailand's most sacred site. Visitors must be properly dressed before being allowed entry to the temple. Men must wear long pants and shirts with sleeves -- no tank tops. If you're wearing sandals or flip-flops you must wear socks (in other words, no bare feet.) Women must be similarly modestly dressed. No see-through clothes, bare shoulders, etc. If you show up at the front gate improperly dressed, there is a booth near the entry that can provide clothes to cover you up properly. You must leave your passport or credit card as security.

Would not be seperated from my credit card though.

In my experience, 3/4 length shorts and tee-shirt or at least shoulders covered with shawl,would be fine for women.
For men, 3/4 pants may be ok if the style is not "combat or too sporty". Play safe and wear long. Also no vests so t/shirt should be ok, better still short sleeved collared shirt.
As for footwear, anything bar flip flops, never known problem with sandals or trainers.
Eddie.

Bunny, do a google for ''correct dress for grand palace thailand'' gives you lay out and tips.

that is pretty much what we found. Skirts have to cover the knees. 3/4 length trousers are ok. Shoulders need to be covered. We were told by a Thai guide that we should wear shoes with a back to them.
One thing they dont tell you- take a brolly. You need it for the shade as it can be very hot there.

Hi we went last year in February and I wore crop trousers and a top with short sleeves, my husband wore long trousers and shirt, we both wore leather flip flops on our feet as were told that the restrictions on footwear was lifted by out travel rep, as they want to show off the palace without so many restrictions and you have to take footwear off to go in many of the places any way.We had no problems and many people had sandals and leather flip flops on.