I am reading with interest for when DD does her Gaisce (President's award) here. I wonder can you do DoE in Ireland?
She's moving up to Scouts next year though, so will be reasonably well versed in most of the requirements - her troupe are reasonably into letting the Scouts organize themselves and Leaders only provide necessary guidance (they stay in Scouts through Venturer years too - so 12 to 18, hence Patrol Leaders are reasonably responsible!).
However, from family and Cub events (and remembering my own Scouting/Guiding days):
The half jar of peanut butter with bits sounds good - if your DC is not into peanut butter (or almond butter), just put in a bag of mixed raisins (and other dried fruit of preference), nuts, and M&Ms or similar to nibble on as they walk.
A handful of boiled sweets is useful too for energy while getting water heated to cook.
John west now have nice tins of tuna with seasonings - not just the salads but like lemon and thyme, or chilli, or basil and tomato - which could be good on pitta or packets of crackers (if they don't crumble).
Ziploc bags are sooo handy for stuff. Ikea do a good range of sizes, but supermarket ones should be ok too. Small ones for teabags, seasonings etc. Larger ones for a full change of clothes, or for the spare socks/tshirt combo to just freshen up - rather than digging through everything to find individual stuff. Also, as secondary wrapped, better chance of staying dry if bad weather.
Definitely use a rucksack liner - decent black sack is fine. I tend to recommend to Cub parents to line the bag with a black sack, but send a second black sack/plastic bag for dirty gear as that's often wet/muddy as well.
Noodles are a good option, another easy food on the go is couscous as it just needs hot water (into the bowl, add hot water, plate over top method someone already mentioned). If using a Ziploc bag to hold it, you can add some seasoning when packing (eg. part of a crumbled stock cube, some dried herbs, dried chilli, (cumin is nice but teens may not fancy that), or just salt and pepper).
Individual custard pots are great. Our Cub Leaders asked for the old fashioned swiss roll (the old gateau type with fake cream in the middle) and custard on our last outing but the mum doing the shopping wasn't ever a scout/guide so didn't understand what we wanted. So we are going back to the regular individual apple pies (Lidl do nice ones!) and custard next time.
If he drinks tea or coffee, a few teabags or the sachets of dried coffee (like you'd get in a hotel bedroom) would be really handy, and a sachet or 2 of hot chocolate powder. Often a hot drink really helps if tired, worn out, homesick, can't sleep, loads of reasons! I'd put those in a small Ziploc, possibly stuffed into the mug.
Headtorch and a small hand torch.
A travel towel - much smaller, lighter, and will do what's needed, can also hang off pack to dry as they walk if desired. Hand towel or just a medium one is probably large enough - I wouldn't see the need for the bath sheet size! And get travel sized bottles (not even 100ml - just 30ml or 50ml would be plenty) for any toiletries that are deemed necessary, but toothpaste/brush, plastic comb, shower gel and deodorant would do it for me.
Spare socks - to change once they arrive in the evening to prevent blisters by staying in damp socks, and another fresh pair in the morning (or yesterday's dried off overnight).
Layers for clothing - to be able to easily accommodate changes in weather, and to add layers once the evening comes on. A woolly hat to wear in bed in the tent (I assume camping?).