Fine for older children who can entertain themselves with a bicycle and a river, and honestly hard work with toddlers: there are no railings, no lifeguards, no pharmacy and no hospital.
Don Khong is the easier island
The biggest island in the archipelago, reached without a long boat trip, with a road round it, proper hotels and far fewer backpackers. The official tourism board calls it the easiest of the islands to reach; with young children it is the sane choice over Don Det.
Khone Phapheng is the manageable waterfall
Unlike Li Phi's cliff walkways, the mainland falls have paved paths, shade, railings, food stalls and toilets, which makes them the one big sight you can do with small children without permanent vigilance. Go by tuk-tuk from Nakasang; allow half a day.
Bicycles, and what to bring for them
The island lanes are flat, unpaved and effectively traffic-free, which is why everyone cycles. Child seats and helmets are not something you will find here, so bring your own if you need them, or hire a tuk-tuk for the longer runs and let the children walk the short ones.
Life jackets, every single crossing
Longtails are the local bus service and boatmen do not automatically hand out jackets. Ask, and do not get on without them for children. The Mekong here has strong, sudden currents even where it looks flat, and the rapids at Li Phi kill people.
There is no hospital out here
The nearest proper medical care is in Pakse, roughly three hours away by road, and the islands have at most a small clinic. Bring a first-aid kit, oral rehydration salts, repellent and any prescription medicine you might need, and take travel insurance that will actually pay for an evacuation.