Your fast is valid. Bad thoughts which annoy you will not harm your faith nor your worshipping Allah.
Repeating Isteghfaar and repeating Salawaat and repeating La Hawla wala Qowwata ILLA Billah and reciting Sura Tawheed is very useful always especially in driving away satanic thoughts.
If the 10 year old is a female, who has reached her shar'i age of bulugh, then yes, she will need to pay the kaffarah of 'feeding 60 poor', along with doing the qadha' of that day.
Fasting during the whole month of Ramadan is obligatory on every Muslim even if they don't know how to perform Prayers properly. You must fast and pray as much as you know and must learn as much as you can.
You should take help from those Muslims who know how to pray or pray with congregation.
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