PICU Attending here...
Not an easy situation to be sure. I'll have to find it but there is a good study (with really large sample size for a pediatric study) with 29-60 day olds with confirmed bacterial UTI from a couple years ago. #1 predictor of severe disease (I believe defined as bacteremia/meningitis) was clinical judgement of the provider if they looked sick/not sick. Complications in well appearing kids were extremely rare (less than 1% if I remember correctly). Again, these were babies with confirmed bacterial infection.
Based on that, I typically recommend that anyone under 4 weeks gets everything: LP, Amp/cefotax, admit, etc.
4-6 weeks, if ill appearing gets everything. If clinically looks fantastic, will do blood/urine/amp/cefotax and admit, but no LP. However if they so much as look at me crossways, they get a needle in the back. Obviously, ideally you want LP before abx, but we can deal with it if not. There is still pleocytosis to note and while we threaten 21 days of abx when parents refuse the LP, it's more often than not about clinical course for abx duration.
I would be more cautious than CHOP, mostly because it's impossible to guarantee the degree of follow up that they can.
After 6 weeks, if well appearing AND have reliable parents, blood/urine, no abx, close follow up. If parents are unreliable or there's any question, admit, no abx and watchful waiting.
It's worth noting that ceftriaxone does cause calcium issues in newborns, so it's best to avoid that until after 2 months, hence the cefotaxime