In 2013 we hosted our then 'Nearly 5' and 'Just Turned 8' Granddaughters and in 2014 it was our 9 year old Grandson.  None of them were particularly happy with long scenic drives so we opted for shorter, more active options.

In BOTH years, anything Animal Related was a BIG hit -- a Sheep Farm, horse riding, so I feel confident suggesting that Petting Zoos and / or Animal Sanctuaries are probably safe bets.  Late April is lambing season, as well -- should be LOTS of cute, baby sheep about!

In nearby Ballyshannon:  http://openfarm.ie/farm-safety/facilities-info/

Further afield:  https://www.yelp.ie/biz/donegal-donkey-sanctuary-castledowey?osq=Animal+Rescue+%26+Shelters       A donkey rescue sanctuary near Letterkenny.

                       http://www.eaglesflying.com/   Irish Raptor Research Center   About 1/2 way between Knock and Sligo, just off the N17.  They also have a petting zoo.

This Bundoran-specific site might be useful:  http://discoverbundoran.com/for-the-kids/   

Whilst the Adventure Park,   https://bundoranadventurepark.com/location  and Waterworld (fully indoors!)   http://www.waterworldbundoran.com/     will likely be hits with the kids, there are also some nice attractions for the older crowd:  https://www.discoverireland.ie/Arts-Culture-Heritage/kinlough-folk-museum/45195/  

These should get you started!