The nice thing about doing it in the UK is that because there tends to be an older clientele, so much has already been thought about for you, playing cards, spare blankets - even things like baking tins in case you fancy making a cake and a full tea set for afternoon tea.
I would check the inventory to be sure and ask if they allow you to order a grocery delivery to put in the boat before you arrive. Makes such a difference to not have to bother with that when you bring your bits on board.
It can be cold and a bit damp at night, and it can be smelly if the toilets go into a tank you have to pump out, so bring an air-freshener or incense. Bring a spare quick-drying blanket if you have one.
Make sure you have raincoats, sunhats and sunglasses and warm fleeces. Make sure everyone has footwear that grips properly.
The thing about the boat is your itinerary- how far you plan to drive the boat. With children and a dog you will need to make full use of any places you can get out and have a run-around, because you can’t move around inside, so you may need to do a less ambitious itinerary, involving less cruising with longer stops.
If you have them, bring binoculars- very useful for wildlife as well as making out landmarks at a distance.
Bring things to keep the kids and dog amused indoors if need to cruise in the rain.
Plan everything with a good margin for error, so you don’t get stressed needing to reach your planned mooring in the afternoon.
Have a lovely, relaxing time.