The safest browser for kids are dedicated kid safe browsers like Kidzui, SPIN Safe Browser and KidSplorer browser.
The kidzui browser is a straightforward dedicated kid browser with a curated database of age appropriate content and also strong filtering that is based on parent set age groups.
SPIN Safe Browser is also a dedicated kid browser made for kids and it blocks explicit content and also enforces safe search on major search engines.
SPIN Safe Browser also integrates with Boomerang Parental Control and allows parents to review web history and customize blocked sites.
KidSplorer browser is also another dedicated kid safe browser that allows parents to create a whitelist of approved websites for their kids to visit and browse, so children can access only websites that parents specify and allow.
Other browsers that can be safe for kids are Safari web browser, Chrome with Google Family Link and even FireFox.
Mozilla is also working on a kids mode for the FireFox web browser to help make the browsing safer for children using FireFox web browser.
Chrome with Google Family Link allows parents to set up a child's Google Account with parental controls through the Family Link App.
And for Safari you can use the built in restrictions features in iOS settings to enable parental controls on Safari, block specific content and prevent new app downloads.
Also a kid safe search engine is Kiddle, which is a visual search engine made and designed specifically for kids.