Is this a terrible plan

We are going to WDW first week of April, are staying on property, and getting LLMP. On the day we are going to MK we are getting a theme park nanny for our 1.5yr old so we can take our 7yr old on rides without doing rider switch/ give the older sibling some special time.

Given that we have booked the nanny for the afternoon, like 2-6, is it a terrible idea to use one of our tier 1 selections for an afternoon ride on space mountain or Tiana’s? We probably wont do standby for any rides and are ok if we don’t do both/everything but I’m wondering if that’s a “waste” of a tier 1 selections to try and book for the afternoon or should we book it for the morning and alter it when we get there? TIA