natural as used for children has changed meanings - in later wills almost certainly refers to illegitimate children - however in 18th C and early usually refers to those 'naturals' - today we would describe them as having learning difficulties (OED quotes this use for half witted from 1553) - the early 15-16th C use of natural as eg not adopted had given way by 1600 to mean illeg (see entries in OED)