Advice on tyres for a 109 mile charity cycle
Variations in this question must be one of the most asked on this site, but I can't find a good question and answer to direct you to, so I'll just answer it.

There are many rim width to tire width charts that can be looked up on the Internet. An example is on this Sheldon Brown page. It's highly unlikely that the tires supplied with the bikes are at the narrowest limit of the rims, so 1.5" tires will be fine.

If you are looking to decrease rolling resistance, go for a road oriented tire with minimal tread rather than an on/off road hybrid. Inflate to the upper end of the specified pressure range.

It goes without saying that you should have the bikes checked over and make sure the brakes and gears are properly adjusted. Also, make sure the drivetrain is super clean and properly lubed - that does actually net an improvement in efficiency.