Why a 10 year visitor visa? This would still only permit a maximum stay of 6 months and your cousin wouldn't be able to spend more time in the UK than he/she is spending her home country.
Child care, even when unpaid and for family
is not permitted while on a visitor visa as it is considered as working.
What evidence will your cousin submit of financial self-sufficiency and strong ties to his/her home country
Not that it's significant, but there's come confusion...you mention 'nephew' and then refer to 'she'.