It depends really.
Ant weights are a great way for new young builders to start of in as they will get some hands on building and many of the components are simpliler. Apart from modding the servos which i sugest you do an ant weight can just require two servos a battery and rx.
Antweight events however just run ant weights so more to your calinder and traveling more. However the next big robots live event is holding ants

With featherweight you would mostly have to do the work. I doubt you would want your son holding anything dangerous.
However featherweights are very popular and all events that hold heavys will usually have feathers aswell.
To be quite honest feather would suit you more. Bad thing to say as I have done ants for a good 7 years. Possibly give both a go. Ant weights dont require much and I bet you all ready have the parts around to build one.