That looks exciting!

Take some toys, something for LO to sit on when it's wet, water if you don't have running water, soap, old towel, change of clothing for LO, snacks and drink, minor first aid stuff, and (as if you don't know) clean nappies because "going to the allotment" translates as "you can have a poo on the way". I painted the inside of my shed door with black board paint and took some chalks which was fun for quite a while.
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Edited to add that the thing I keep in the shed at the allotment that I wouldn't be without is an old parasol. Also, if you get the chance to collect sample sachets of sun cream, grab them while you can and keep them there too.