Kate Stevens, who had travelled from her home in Canterbury, Kent, was with her son who lives in in Enfield, east London.
She said they were both were denied entry.
Ms Stevens said she bought the voucher in November and Jacob had been wanting to see the show for some time.
She said: “[My son] was standing next to me and then I could see other people being told and I went up to somebody, and I showed him my ticket, and I was like: ‘Does that mean us as well?’ And he said: ‘Unfortunately, yes.’
“[My son] just burst into tears and then got really upset and said: ‘I just want to go home, I just don’t want to be here.’”