Cost of lending out a child's bike? My son's elementary school teaches kids to ride a bike in gym class. Basically, they adjust the schedule so the 2nd grade kids (7-8 year olds) have extra gym classes during a 1 month period and during these classes the kids are taught to ride. The school (technically the PTA and more technically an off shot of the PTA) owns 30 pedal bikes, 30 balance bikes, and 30 sets of safety gear so that there is a bike for every kid in the class. The PTA has arranged with the local bike shop to service the bikes during the first class (the PTA pays for parts and the bike store donates the labor). The bikes sit unused for 11 months of the year. Our school is very large and the bikes are essentially in use 5 hours a day for the entire month.

There is talk of loaning the bikes and safety gear to other schools so that they can offer bike riding lessons also. The concern of the PTA is that this will cause the bikes to have a shorter life span and additional maintenance costs. What is the best way to estimate the cost of loaning/renting the bikes to the other schools? Our PTA is quite good at circumventing rules/laws so the legality of rental payments and liability are not a concern.

[UPDATE: We are now in the second year of lending the bikes out. Last year we lent the bikes to 2 other area schools and this year the plan is to lend them to 4 school. The final activity is for a group of the kids to ride the bikes to the new school to hand them off. The other school returns the bikes in the same way. Our LBS had no problem with the arrangement. The deal with the other schools is they need to work with an LBS to do a safety check and pay for anything that they break. We decided on no money and they seem appreciative. So far it has been a win all around.
The marginal cost of cycling is extremely low (i.e. the cost for an extra bit of use), especially as kids' lessons are going to be light use (I doubt they're hurtling off kerbs, for example). Kids' bikes are also heavily built in proportion to their riders, who are weak compared to adult riders, while the chain (for example) is unchanged.

Depreciation of bikes has little to do with distance travelled and more to do with age (unaffected by the loan) and apparent condition (e.g. rust - how are they stored? How often are they dropped on painted parts?).

If the bikes are being serviced once a year anyway, that's probably enough for some extra riding, though they may want a safety/basic check after being transported between sites (have the brakes been knocked, is there air in the tyres...?).

Ultimately this is a form of business decision (though I dislike the term in this context). It's highly unlikely for the extra wear to shorten the life of non-consumable parts, so if you covered your consumables costs (with a possible allowance for extra wear) you should be fine. Accident damage is another matter (also theft etc.). I suggest you enquire about insurance but probably choose to self insure. 

You should talk to the LBS volunteers to get an idea of whether they think they'll need to do more, and whether they'd be happy to do so on the same terms.

So to actually estimate what it might cost, I'd start from an average consumables cost per bike per year from your records. This is for a month's use, so I'd try to recoup that per month lent out. This is almost certainly more than enough alone as rust and perishing rubber are a function of time, but if the number you get from that seems low in the context of the market (sorry!), a safety factor wouldn't be a bad idea. Obviously if you do end up with (extra) insurance costs you'd factor this in, and you'd gauge interest in advance to know how to share that between renters.