Went to a fete at the weekend with my friend and her 7yo and 3yo, who I help look after sometimes. The younger one chose a 50p toy from a second hand stall. There were two similar toys, and 7yo was asked whether he wanted the other one and he said no. Everyone had a cake and did some crafting, 7yo was bought a book on a game he likes and was bought candyfloss. The 3yo had been happily playing with his toy throughout. We went to leave and 7yo then said he was jealous of his brother's toy and he did actually want one like it too. Of course by then the matching toy was gone. He was bought another toy from the stall instead but said he didn't want that.
Progressed into a full bore screaming tantrum on the way home about how it was the worst day ever, he didn't like anything, it hadn't been a nice time and he hadn't had anything he wanted. He denied that he'd been asked at the time if he wanted the other toy, screamed "no you didn't!" when his mum said that, tears and shouting the whole way back. After a bit of decompressing at home he was then fine again and later had a go playing with the toy - that's the silly thing about it all really since toys in their house are pretty communal and the 3yo has a short attention span so he was always going to get to play with it!
Any suggestions of how to manage that kind of FOMO and associated tantrums? I tried a bit of jollying of jokingly saying oh yes it was a horrible afternoon wasn't it, eating cake and crafting at a fete, while reassuring that his brother would probably give him a go with the toy later, and that didn't go down well. His mum tends to go for "I can see you're sad about this" and just waiting it out, but it does seem a bit OTT that he's screaming at her that she didn't offer to buy him the toy at the time when she did! Thinking back to my childhood, my parents would have probably said I was being rude and ungrateful and wouldn't get taken to a fete or bought things again if I was going to behave like that. Up to a point I think that probably wouldn't help once you're in a tantrum, but as he gets older should he be encouraged to manage how he's displaying being upset about this sort of FOMO/I want what my brother has/I want a turn now/I want to win? (He is prone to similar when needing to take turns on things at playgrounds, when it's time to leave somewhere fun, etc)