According to a report from researchers at the Johns Hopkins School of Public Health, 54% of gun owners in the US do not have all of their guns stored safely in the home. That includes homes in which younger children are present.
The study used data collected in 2016. Hopefully, practices have improved since then, but perhaps not.
And you wonder why so many kids get killed?
Cassandra K. Crifasi, Mitchell L. Doucette, Emma E. McGinty, Daniel W. Webster, Colleen L. Barry. Storage Practices of US Gun Owners in 2016. American Journal of Public Health, 2018; e1 DOI: 10.2105/AJPH.2017.304262