I've done bit (I'm a British Canoeing Coach and Instructor)
First thing I'd say is you may struggle finding a family Kayak with 3 seats. Most have a maximum of 2, though with some (I'm thinking of a Perception Kiwi) a 3rd smaller person could fit in the middle.n You might want to think about getting 1 double kayak and a single kayak instead ?
Sit on tops are a good idea for a total beginner as people don't feel worried about being trapped in them if they capsize, the downside is they have very little back support and general (connectivity) which means they aren't as comfortable for longer journeys or sessions and make carrying things (picnic lunches, spare clothes, sunscreen etc) harder. They are just as stable as enclosed Kayaks and some can with the right training and skills even be used on grade 2 /3 whitewater.
Sit in Kayaks are generally a bit more comfortable and better for longer journeys as there is more space to stow things and will keep you a bit dryer if it rains, or warmer if it gets windy or chilly.
If it is just flat river and lake paddling, then you may find an open canoe (think Native american Indian) more suitable as they can easily carry 3 adults or two adults and 2 small kids. They would be a little bit "interesting" trying to get through waves to and from the beach though.
Don't forget safety equipment as everyone needs their own correctly fitting buoyancy aid (can be had for as little as £15 from Decathlon)
https://www.decathlon.co.uk/C-50789-buoyancy-aids or more specialized BA's can cost a lot more :-)
Normally i'd suggest finding a local kayaking club and going down for a try out session and to learn some skills ( actually I do recommend this), but appreciate that at the moment that might not be possible due to COVID-19 and wanting to get on the water now , so take it easy and be careful.
YouTube is a good place to learn some basic strokes to go forwards, back and turn, certainly enough to get you on the water and paddling around.
Alternatively, have you thought about paddle boarding ? It's great fun and kids find it a lot easier to do than adults (better sense of balance and less weight - little sods ;-) ) They can be used on rivers and in the sea and you can even get paddle boards that hold 8 people !
Feel free to ask any more questions