I guess the sentence actually reads:

Il faut nous promettre de ne pas nous réserver autant d’émotions.

The meaning is something like:

You have to promise not to keep so many surprises (thrills?) for us.

or (perhaps?), as suggested by @Lambie

You have to promise us not to harbor so many feelings towards us.

Edit: Alice in Canada being a translation of The message in the Hollow Oak, the original sentence reads:

But you must promise not to provide us with so much excitement.

Here is an excerpt from the original book: