Sadly, I'll admit the 1st thing I'd look into is if there are any regulatory/health and safety/OSHA... rules or regs on what they can or can't be given.
When you say tool kits, it seems you mean just not the tools but also basic supplies for doing experiments?
For electrical I'd throw in some bulbs, led's, maybe even capacitors, solenoids, motors etc. We used to have some 'modular' bits and pieces that you could build circuits with then pull apart.
For mechanical, as well as a bunch of fasteners how about something like a mecano/erector set kit with gear, pulleys...
Make sure and include appropriate metrology equipment, a good steel rule, tape measure, calipers (real Verniers maybe without the digital read out just to make them think), Micrometer, level, feeler gauges... Your budget may not extend to all of these but depending what kind of projects they'll be doing and what they already have available in the physics room... you can down select.
Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484