The rule in Spain, and also, as far as I can tell, in Argentina and Chile, for example, is that children can compete in "Sub-10" or "menores de 10 años" if they are not yet 10 on 31 December of the calendar year preceding the competition, or to put it another way, they can compete in any competition held before 31 December of the calendar year in which they reach the age of 10. So many children who are already 10 can compete in these competitions, but many others can't (depending on when their birthday is and at what point in the year the competition is taking place).
The rule for 10 and under is normally that you can compete until the first day of the month in which you reach your 11th birthday. So virtually all 10-year-olds can compete in 10 and under.
The ITF uses "10 años y menores" in its Spanish-language rules, and I would personally be inclined to use that here, even though many Spanish speakers would probably assume it means Sub-10.
http://www.itftennis.com/media/194279/194279.pdfSub-11 would be open to quite a lot of children who are already 11, which is certainly not true of 10 and under, so that wouldn't be suitable.
peer agreement (net): +3