We were in Kempervennen a few years ago and really enjoyed it. Our youngest was three and we had a toddler seat fitted to the back of both adult bikes for her.
ISTR that we added bike hire to our package when we booked online, and this resulted in vouchers when we checked in. We took them to the hire place, selected our bikes and were good to go. They have a wide selection of of trailers and toddler seats, and plentiful numbers. The whole process is very relaxed so you need to get into a continental mindset.
I did sign my littlest up for the Wannabee/When I am older sessions and she enjoyed them. Again, I added these to the shopping cart when booking, and was given vouchers at check-in. We also paid for several activities, which we finally booked in person when we arrived.
On the food front, we booked a package of four meals - a breakfast buffet, dinner buffet, pancake lunch and cottage meal. There was a discount with this package but the best thing was that the cottage meal (fondue/raclette) lasted us about three meals with all the leftover meat and salad. We had 2 or 3 more meals out, and the rest we self-catered. I thought the market place was very reasonably priced considering there was no reasonable competition.
We had a 'deluxe' cottage (the middle of the three classes) and we found that it was child-friendly. The KV cottages are all on one level so you don't have to worry about stairs. There was a travel cot and high-chair in the hall cupboard. A lot of the plots at KV are on the side of small streams, so you may need to be extra vigilant about the water and the super-friendly ducks. If you were take anything extra, I would say plastic dishes and utensils for the children. I think they provide cleaning supplies, but a roll of paper towels wouldn't go amiss.
There are plenty of free things to do with little ones. There is a petting zoo, a small beach, a lovely playground as well as the swimming. If your cottage is anywhere near the sports centre, you will be right in the centre of these activities.
For yourselves, the really special thing about KV is that snowskiing and waterskiing. If you are inclined to do either, then do give them a go. Unfortunately they have no creche, so it would be a case of one doing and the other watching.