2.5 year olds can be quite a handful. Eight week old babies mostly sleep. I suggest that your son-in-law and daughter would quite like an easy day out for her birthday and that's why they don't want a two and a half year old in tow.
To put it another way, they are taking advantage (and why not?) of the usual arrangement for Mondays.
Don't over think it.
How to Keep Living at Home Longer


