Nigellas cherry chocolate cup cakes are divine, and really easy to make. You can make them with gluten free flour, which would give added appeal as theres never anything that gf children/adults can have from cake stalls normally.
I'm a fan of traybakes for this type of thing. How about Bakewell hearts? Make a big sheet of Bakewell, ice it then stamp out heart shapes using a cookie cutter. Those little heart shaped sprinkles look lovely on top. When you're sick of eating the offcuts (this can happen, believe it or not), they make a nice pudding with some custard.
l;emon flavouring is found with allt he baking stuff, alongside vanilla essence etc. I don't think you could use juice, as it wouldn't be concentrated enough so the amount you would need to use would make the icing runny.
i make fish cakes for any cake stall events and they go really quick. Fairy cakes, cover in melted white chocolate break some fan wafers in half and use half to make a fish tail. Put white chocolate buttons on to make scales. Abit more white chocolate on the tails and some rainbow sprinkles. Easy and effective!