Bitdefender Free
Sophos Free

Both quite good. Ive run both on my work PC.
The dont try & con you into buying crap addons like some other freebies , big issue with popular freebies: endless popups and a bloated product

pay AV can fail to detect just as badly as many freebies, sometimes Pay AV is worse than freebies for detection and REMOVAL. .from experience
Ive seen malware get past every major AV product over the years .

MS have a very poor history with their (home) AV products, very poor over the past 10+ years . It may be Ok this month, but it 2 months time ?????

Saying that, common sense and being carefull is the best defense, kids arnt known for that