I love Alice in Wonderland! I actually had a small teaparty at one point but mine was for adults. Asda has a range in at the moment with lots of table and cooking ware that may be good? I bought some last week.
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Some ideas, if you've been on Pinterest I may just be repeating stuff you have already seen.
Jammy dodgers would be a nice easy one.
Pink wafer biscuits with purple icing stripes to make them cheshire cat themed.
Playing card checkerboard sandwiches.
Different themed cake pops? Serve the cake in large teacups maybe with ice cream. Or just fill the teacups with sweets on the table.
Give the non birthday guests a merry unbirthday card
Little cherry tomato and cheese made into little toadstools. Or maybe strawberry and marshmallow ones.
Mini pizzas or savoury pastries cut into shapes like rabbits, flamingos, flowers etc.
Milk bottles like these for juice, mocktails or milkshakes with drink me labels.
Bottles from The Range
Have them decorate and make their own mad hatter tophats.
Set up a game of croquet for them. You can get an Alice themed one on Amazon but it's £45 and you could probably make a version yourself fairly cheaply..