Regarding the travels between the UK and mainland Europe you should make the seat reservations for the Eurostar ASAP. 

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

You may need a plan B if Eurostar is full for passholder fares. Options are book full price tickets. Fly to Paris. Travel by boat via Calais but this is not recommended!

As you’ve been told book now! You’ll probably have difficulty with Thalys trains from Paris to Brussels as well but there are alternatives. The community experts can help if you get stuck.

A one month pass looks ideal.

One option B for E* is to travel to/from Lille and then a TGV to Paris (takes about 1.5 hrs) with res costs of 10 or 20 euro for TGV.

I count 7 or 8 traveldays, so a pass for that nr would be sufficient, unless you are also determined to use trains in between.  Short trips like BRU-Bru are probably cheaper bought locally-there may even be family discount. Travel on 1/1 is not really recommended.

You should calculate the cost of booking these through the operators as regular tickets, there are really only 6 days of that itinerary you would likely get value out of eurail passes and 3 of them will be on high reservation fee trains, booking regular tickets early enough may be cheaper.

The main reason passes are going to be reasonably competitive is due to the 2 kids being eligible for free passes.

If you do decide a pass is best then 7day in 1 month should be more than enough.

Also Berlin-London-Littleport is a very long trip, well over 12 hours and multiple connections, to take the day before you fly home.

I had to google Littleport, a village near Ely? If so I would say flying Berlin - London Stansted would be a far better option for this journey, Ryanair have an afternoon flight currently at €25/seat.

There is a mid ground between getting a pass with just enough travel days (7 in a month 335 euro) and a full month pass (670 euro).

The 10 day in 2 months pass is 401 euro and gives you some extra travel days for local journeys or just in case your travel goes a bit pear-shaped. There is nothing stopping you using only 8 or 9 in one month.

Thank you. 

We've taken Berlin out - and booked up our pass and London to Paris seats. 

Thanks team. REALLY appreciate the help. 

Will now look at the other seat reservations.