What a lovely story, Phil. Good on you, kids.
I hope the Hatsan delivers many smiles and happy times.
I've never owned one so can't come up with any specific recommendations, but trying a selection of pellets from known quality makes is always the way to go. The Air Arms Fields mentioned above would be on many people's short list. 8.44 grains in .177. With it being a PCP I think I'd want to be trying the JSB Exact Heavy as well as the normal Exacts. And I'd most certainly be testing the H&N Field Target Trophy and Baracuda FT. The QYS pellets are very much in favour currently, also.