Assalamualakium Ustadh my sister she is 10 years old and she accepted Islam a few months ago and she is gone to a family trip she will miss the start of Ramadan, but she said she wanted to fast so if she comes back will she be able to fast and get rewarded even if she did not start on the first day

Ustadh AbdulAziz al-Haqqan

Yes as long as she did not hit puberty.