My idea is that Nagato is humiliating/babying Warspite, with Warspite being dethroned from her battleship/chair/throne to a booster seat. Depending on the design you use she could be strapped in to the seat or just stuck in it. Up to you whether she is padded or not, whichever seems more embarrassing for her.
A pic from patreons Fanart Friday - This one was two weeks ago I think ~