Honestly - for a sandbox for your kids I wouldn't use that stuff.
You really have white pine? That's reasonbly rot resistant. Just make sure it can dry out, but honestly, does it need to last more than 5-10 years?
Use a stain. An oil stain. That will keep the moisture from soaking in and the pigments will protect from UV. Ground contact will always be a problem but less so if its not on a low spot.
Very little construction lumber is made of white pine. Mostly used for boards, and even there its limited.
Look at the stamp on the lumber and you'll know what group of species it is. SPF is spruce pine fir. Hem-Fir is mostly Hemlock. SYP is Southern Yellow Pine.