The children will love Gt Y. (Cromer on the other hand is for the older generation)
A day on the beach, a day at Pleasurewood Hills, another day on the seafront in the amusements, on Britannia Pier and up and down Regent Road, Africa Live (zoo), a day inland at Bewilderwood, a day inland on the Norfolk Broads at Wroxham and Wroxham Barns, another day at the seafront (!) this time the model village and the pleasure beach, a day at Redwings Horse Sanctuary, a train ride into Norwich and back, all great fun for kids and not hugely expensive.
Yes parts of Yarmouth are run down, but go with the spirit of the traditional seaside holiday, see it from a kids point of view and with a sunny day and a bag of chips, you'll have a good time.