At my daughter's school, they had that project week, too. Easily the most important experience in grade school IMO. They put the liquid inside a tube with a non-return valve. Also, all-time adult supervision, clear instructions upfront and kids did not get the tubes until they needed it. Kids were 8/9 years old. Yes, you have a significant risk and I am very glad they took it to help the kids grow up to be responsible people who can take limited risks.