We did Dalbeattie for similar reasons as you, have also done similar in Northumberland.
If the little lad is into trains, have a search for Cynfal Farm (possible Welsh spellings issue!) which is near Tywyn – nice cottages, but the farm has it’s own halt on the little steam railway that Rev Awdry based his mountain trains on….thought my lad would burst when he discovered he could just hail a steam train and it would stop for him! Also handy for beach and trail centres, and you’d be surprised how interesting the CAT at Mucknfluff can be to toddlers – ours are going back for the third time at Easter (now aged 8 and 5)
Also the North York Moors are great – Dalby, Whitby and Filey on the coast (Scarborough is a dump!), North York Moors railway, the train museum at York, brewery at Cropton, chocolate factory at the next village down who’s name escapes me at the moment…..
edit: that reads wrong, I know Dalby isn’t on the coast!