There are lot's of 'leaflets'on the net from reputable sources that suggest using toes for infants and toddlers is fine and normal practice
https://www.childrensmn.org/Manuals/PFS/HomeCare/018321.pdf
The Great Ormond Street guidelines for professionals also includes using toes as well as fingers for children over a year ; but warns to avoid thumb and forefinger . (because needed for fine dexterity)
http://www.gosh.nhs.uk/health-professionals/clinical-guidelines/blood-glucose-monitoring
Most say not to test toes in adults. I'm sure that's for the reason INDY suggests but it could also be because the level might not be the same in the fingers and toes . I don't know whether that;s true but certainly in Adults alternate site testing produces results with a time lag ie the level in an arm or a calf is twenty to thirty minutes behind that of the finger tip. That's OK for normal testing but not good in the case of fast falling glucose.
https://www.bd.com/us/diabetes/blood-glucose-monitoring/how-to-test/alternate-site/
Many finger prickers come with a sort of extra head that's supposed to be used for alternative site testing