OK, so you already pretty much have the answer to your question here in the quote above.
I just have a couple of small points to add:
A split stay does not have to be half of your time at one and half at the other. You can use any number of your nights at one resort and then move for the remainder at your second resort. I would consider how many days that I wanted to spend at the MK vs the other theme parks and the pool day. Then I would book the CR for the MK days and the BC for the other days. (Personally, I usually like to stay for a minimum of three nights at a resort, so, in your situation, I would probably book three nights at the CR and 5 nights at the BC. Just my opinion, however.)
Consider when you want to visit which theme parks, so, if you want to start with the MK, for example, you would book the CR first and then book the BC for the second portion of your stay. Potentially a good day for the pool day would be the day that you move to the BC. (Some people like to hang around the second resorts pool while they wait for the room to be ready.) On that note, remember that check out time of the first resort is 11:00 am and check in time at the second resort is not until 3:00 pm. Sometimes, you can request a late check out for 12:00 or 1:00 pm and sometimes you will be lucky with the room at your second resort ready early, but typically there is at least a period of a few hours when you are 'homeless'. There are different strategies for dealing with this. One is to give your bags to Bell Services (to move for you to the second resort) while you head to a theme park for the day. AT the end of the day you go to resort number two, check in and call bell services to bring your bags to your room. This is easy, but you are separated from your 'stuff' until some undetermined time, sometimes into the evening when Bell Services get your bags moved over for you. Another option is to bring your bags with you if you have a car, check in and then go about your day either to a park or by staying at the pool. This can also be done using Uber, Lyft or a Taxi for a small fare. You can store your bags with Bell Services if your room isn't ready when you check in. The advantage with this is that you can always access your stuff, as Bell Services will bring anything out if you need it through the day, even if your room isn't ready yet, and put it back in storage when you are done.
One thing (unrelated to split stay) to remember with a family of 5 is that the CR rooms are significantly larger than the BC rooms. So, the CR will have two queen beds and a large daybed in every room. Some BC rooms only have the two queen beds, basically because these rooms are even smaller than the BC rooms with a sofa/daybed as well. And, of course, many BC rooms have a small, one person, standing room only balcony. All of the tower rooms in the CR have full large balconies. EXCEPT, be aware that the garden wing second and third floor rooms do not have balconies at the CR.( The ground floor rooms in the GW do have large patios.) If you are looking for lots of room to spread out, the CR in the tower is your best option.
Wherever you choose to stay, have a great trip!