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It may be useful to add half- and full-adder symbols to the macros.tex file (example here), as the existing adder is not particularly useful.

See, for example, page 802 (14 of 20) in this pdf, where at a minimum, the use of the Sigma (Σ) in a block diagram plus a few I/O pins can designate an adder.

Make handouts for participants, including such elements as:

• binary to decimal conversion
• blank truth tables
• binary addition problems
• two's complement
• one-bit adder (with carry-in) diagram, with varying inputs and blanks for the outputs (a variant of the truth table, but with circuit symbols)

Surface mount components can be used to make:

• half adder (one XOR gate and one AND gate)
• full adder (two XOR gates, two AND gates, and one OR gate)
• an AND gate cascade of four (2 + 1 cascade of gates) or eight initial positions (4 + 2 + 1 cascade of gates), to be paired with some XNOR gates (probably soldered in pairs or quads just to make things easier)

• Fab the OR, AND, NOR, and NAND gate PCBs
• order the components
• test that they work as expected
• ensure they don't draw too much current per gate
• build a composite gate like an XOR
• ensure that kids can solder the gates without difficulty

To add or compare two numbers, it will be necessary to have toggle switches for inputs and LEDs for outputs. Having a handful of four-toggle deck PCBs, plus four LEDs (to show which switches are 1 and which are 0) will be useful. LED outputs (five, or a bank of 4, 4, and 5 -- the extra for overflow) on a separate PCB will also be very useful, because wiring up LEDs is annoying and slow. While it is useful to show the kids that you can wire up an LED the wrong way and get nothing, they don't learn anything new by wiring up 13 LEDs and 13 resistors and lots of loose connecting wires. Thus, the LED output banks should be SMT components for sure, and while it will be necessary to solder the toggle switches by hand, the LEDs and resistors on the toggle banks should also use SMT components.

Recent changes in Eagle CAD and its terms of use make it difficult to recommend. It's time to migrate the PCB designs to an excellent open-source software package, KiCAD.