TLDR Topical minoxidil can cause excessive hair growth in children, so use it carefully.
In 2016, a case report from Brazil documented the first instance of generalized hypertrichosis in a 5-year-old girl following the use of topical minoxidil 5% (20 drops a day) for hair loss. The child developed substantial hair growth on her back, face, and limbs, and her eyelashes became longer after two months of treatment. Laboratory tests ruled out hyperandrogenism and thyroid dysfunction. The report highlighted that while minoxidil is an FDA-approved treatment for androgenetic alopecia, its mechanism of action is not fully understood, and it is known to cause hypertrichosis more frequently in women than men. The case suggested that systemic absorption of minoxidil might vary among individuals, and children could be more susceptible to its systemic effects due to their lower body weight and greater body surface area relative to weight. The authors concluded that caution should be exercised when using topical minoxidil in children.
13 citations,
March 2014 in “Pediatric Dermatology” Minoxidil 5% solution significantly improved hair density in a girl with loose anagen hair syndrome over 28 months, with no side effects.
17 citations,
October 2013 in “F1000Research” A 2-year-old boy grew excessive hair after using minoxidil for hair loss, but it improved when the treatment stopped.
212 citations,
September 2008 in “Journal of The American Academy of Dermatology” Minoxidil and finasteride treat hair loss in men, while minoxidil treats hair loss in women.
58 citations,
January 2007 in “Dermatology” Minoxidil use in children may cause heart issues.
29 citations,
December 2003 in “Teratology” Minoxidil use during pregnancy may cause fetal harm.
63 citations,
May 2003 in “Journal of The European Academy of Dermatology and Venereology” Minoxidil use increases facial hair growth in females, more in older users.
44 citations,
January 1997 in “British Journal of Dermatology” 
52 citations,
February 1985 in “Archives of Dermatology” Minoxidil absorbs poorly through skin, with low risk of side effects at 1-2% concentration.
1 citations,
August 2021 in “Cosmoderma” Low-dose oral minoxidil is effective and well-tolerated for treating hair loss.
4 citations,
January 2017 in “Indian Dermatology Online Journal” A child used a hair growth lotion for hair loss, but it caused excessive hair growth on his face and neck instead.
17 citations,
October 2013 in “F1000Research” A 2-year-old boy grew excessive hair after using minoxidil for hair loss, but it improved when the treatment stopped.
44 citations,
January 1997 in “British Journal of Dermatology” 
20 citations,
March 1994 in “Clinical and Experimental Dermatology” Too much topical minoxidil can cause excessive hair growth, but stopping treatment reverses it.
45 citations,
November 1979 in “British Journal of Dermatology” Minoxidil causes excessive hair growth in almost all patients.
26 citations,
April 1977 in “PubMed” Minoxidil, a blood pressure drug, can cause excessive hair growth, but this can be successfully treated with a hair removal agent called calcium thioglycolate.
