HI M.:
I have two children, ages 4 and 3. When my 4 year old was a baby -- we started him on many foods that (in hindsight) he probably should not have had ... eggs at 8 months, peanut butter at one year, etc. with no problems whatsoever. Then, when my daughter was 11 months, she wouldn't eat much and she was petite. I was always trying to persuade her to eat. I tried to give her foods high in calories, to try and bring her up a little higher on the growth-chart. One night, after not touching a thing on her plate, again. I gave her bites of a bagel spread with peanut butter. Less than 20 minutes later, I found myself with her in urgent care (and now I know I should've called 911). After a LONG ride in the ambulance to Children's Hospital we found out she had an allergy to peanuts and all tree nuts. I don't know if her age caused the allergy -- but, I know now, that if she'd had the nuts later, the reaction may not have been so severe.
We carry an epi pen -- and every time she leaves my sight (like today at Sunday School), she gets a big sticker she wears on her back, "no nuts please, I'm allergic". Halloween is difficult, baseball games are hard, and visits to friends and relatives homes -- where labels might not be read as diligently are nerve racking. Don't even ask about a family reunion -- where ALL the goodies are baked in peanut butter, or covered in chocolate that "may contain nuts". I'm the mom who runs around with a popsicle for my kid, or a bag of pretzels ... not because I'm healthy -- but, because we don't dare take another chance.
I tell you my story not to scare the dickens out of you -- but, to tell you that I too have one child who can eat anything -- and does. And, one that I must watch all she puts into her mouth, for fear it might send her to the hospital, or worse. In the case of your grandchild, and your daughter-in-law, I say "better safe than sorry".
Take care,
T.