Not only safe but desirable. the only bees which sting on a regular basis (and only then when threatened) are Honey bees.
Use as an educational tool for the little ones to watch the bees going about their work.

I agree, they wont come to harm and it will be good for them to see the bees and learn about them. Lots of good stuff for kids on the internet.

Just letting you know Seaburngirl. And Owdboddy

My sons 8 yr old boy. Is mad on Nature,(with the help of us Grandparents ), his knowledge he absorbs ,teaches us .So of course the Bees are a bonus to for him to watch going in & out of the box

Hi Pammie ,generally bees are non aggressive with the exception of africanised bees in south american and Africa,but thankfully we are safe here.
Honeybees are on the decline and should be encouraged see
https://www.rhs.org.uk/advice/profile?pid=960 i love them wish they would camp in my garden :}

Don't worry about the bees. They won't bother you. Just let them be bees. If they really make you nervous, simply move the bird box someplace else, but that might agitate them.