A split stay means 2 reservations. So that alone would prevent you from carrying over dining credits. Also, to get a dining plan you must purchase tickets. For your one night stay before your cruise, you would have to purchase minimum one-day park tickets, currently $89 each, for everyone in your room. Your family is 4 ppl and that would be $356+tax. Then when you return from your cruise, your second stay with dining plan would require purchase of park tickets again.
The Quick Service plan for one night would get each person 2 CS meals, a snack credit and refillable mug. Today's costs is $34.99/person.
DDP gets you 1 TS, 1 CS, a snack credit and refillable mug, at today's cost of $51.54/person. Next year it'll be more. You have 4 people, so at today's rate, that's $139.96 for QSDP, or $206.16 for DDP, and the dining credits would expire at midnight on the day you check-out of the first reservation.
Seems better to just pay out-of-pocket for your dining during your one-night stay. Then get dining plan with tickets for your second reservation after your cruise.
Sorry if you misunderstood how split stays work. However, your vacation plans sound wonderful!