Help with child car seat in motorhome

I am not an expert on "car" seats but I have in the past considered whether I could carry a grandchild in my 2017 Citroen based van (essentially the same as your Ducato). 

I think a seat for a six year old will be "Booster seat" i.e. a seat with a shell into which the child fits. The seat is restrained by the seat belt. Depending on the seat and childs size and weight the seat may or may not have its on multi point harness or the seat belt may restrain the child as well as the seat itself. 

Its my personal view that I would not use a seat which depends only on the seat belt for restraint. Isofix which connects the seat frame to the seat/vehicle body  gives more security and a top tether as well is good. The forces involved in even a low speed collision are immense. 

So I suggest that you make quite sure that your van does not have isofix  and if it does have either you can source a suitable seat. 

A quick perusal on the internet suggests that some Ducatos may have isofix and it may also be possible to retrofit isofix or replace the seat.

None of these alternatives will be cheap but what price a child's life. 

ROSPA have great guidance on their website and you may find help on other forums such as Out and About (MMM) or Motorhome Fun. 

https://www.rospa.com/road-safety/advice/vehicles/car-seats

I have the same query as have just got a new smaller grand child  who will be six months when on holiday. we need a seat that'll go on the lounge seats of  chausson flash 14. The current inertia real belts do not fit around the seat we use in our car. the fixings are not isofix. Does the club have any reviews on child seats especially for those too small to use booster seats

We have used a booster seat to restrain our 5 year old granddaughter in the dinette  in our Hymer. The seat redirects the seat belt so it is at a suitable level across the her chest. There is a certain amount of movement as the seat belts work on inertia and obviously does not  lock if you just move it about slowly.  However, it fits just as well as it does in our XTrail. If you give it a rapid tug, as would happen in rapid deceleration, it locks immediately. When she was younger / smaller we had a seat with an independent 3 point harness, which fitted into the seat belt and also had a top tether that clipped into a fixing in the boot. That sort  could not be used in our van as there is nowhere for the tether. For that reason we didn’t take her away until this year. The booster seats are not recommended for younger / smaller children.

We have two g'kids that occasionally travel with us, one 3 and one 6. 
the 6 yr old has a booster seat and the three year old has a car seat. The belt has to be threaded around the outside of the seat then back behind it and then out the other side.

the seat belt holds the seat steady and there are belts within the seat that holds the child in the seat.

one thing that makes life awkward is that the two belts on the fwd facing seat clip in on the outside of the seat..ok for the inside child (clip easy to reach) bit for the outside child you have to reach over them semi blind...would be better if the belt clips were both in the middle of the seat as in our small hatchback.

Gov.uk has the rules and legislation re seats. So height/weight requirements for booster seats.