You can get relatively good but cheap boots in places like Mountain Warehouse and Decathlon. They are really worth it.
And a decent pair (or 2) of hiking socks to wear under the boots.
If your dd only gets the one holiday out of them, you could think about donating to a local scout group - they will be well used by someone there.
A small backpack for lunch/water bottles etc, and spare sun cream. Again decathlon has decent small ones very cheaply for kids, or even school bags work ok - just not the bags with drawstring top and straps as they dig in too much into shoulders (I'm a Cub Scout leader and we've banned them as I ended up carrying so many!)
Refillable water bottles, maybe with some squash in it.
When we give Cubs a packed lunch when we're away, it's a sandwich that they will eat ( we offer a choice of ham, sliced cheese, jam or chocolate spread). Along with an apple, pack of crisps and a penguin bar each. And I bring boiled sweets and emergency chocolate in my pack.
A small first aid kit - a few antiseptic wipes and band aids, and an elastic bandage in case if a sprain (can all fit in a ziplock sandwich bag - no need for a fancy kit). And your spare sun cream to reapply at lunch if the weather is bright (not necessarily sunny even) - a single application may be enough on duller and rainy days. I usually tend to bring a. Pocket pack of tissues as well - many uses! And a separate ziplock bag for any rubbish to bring home.
Rain gear - Mac in a sac type jackets, and they do trousers as well which can really help! They can also be useful to sit on at lunch stops, if the ground is damp.
And 1 extra layer per person in the bag - a fleece jumper/jacket, a hoodie etc. because you can get chilled quickly in the breeze when you stop for lunch etc,.
But it's easy to get even small DCs to carry their own rain gear and layer, and probably lunch, if you carry the water (the heavier part).