Rethink
I would honestly be worried about Kellett and its lack of a track record with results as well as university placements. It's not proven to have done well and in all honesty, very few schools have matched ESF in their ability to allow students of all abilities achieve well in exams. While many IB schools in Hong Kong score higher than ESF schools, almost all of which use IB as a selective "stream" for the high-achievers in their school and have very small cohort sizes (think DBS, St. Paul's Co-ed etc). These are kids who have already excelled and have been hand picked to take the IB exams and really their success does not reflect on the educational quality of their host schools. CDNIS and CIS are the only schools with cohort sizes enough to warrant comparison, and both are extremely academically selective whilst ESF schools operate on a waiting list system open to a broader range of students.
If you are concerned of where your child will end up for higher education, it is better to think conservatively and consider only which schools have a good track record. These are the Island Schools (West Island School, South Island School, Island School), King George V School, Chinese International School, German Swiss International School and Hong Kong International School (Also LPCUWC but this is really an international boarding school, not a hong kong international school). Canadian International is one to watch because they have been getting good results, but their students will struggle to get admitted to McGill compared to the aforementioned schools which it is generally very accessible. Likewise, well regarded London-schools are easy targets for these schools mentioned (and top-tier local schools) and high reaches for others. If you are hoping for Ivies, really these few schools plus or minus high performing local schools (DGS often), have a complete monopoly on Hong Kong admittances.
I am sure schools like Kellett have made strides to improve themselves and appear very friendly and open to students and parents alike (on open days etc or on speculation about renovation, buildings etc), but it is not prudent to use this kind of thinking entirely to decide where your child will end up. Kellett simply does not have a proven track record, and many of the new upstart international schools have been floundering for quite some time (ISF for instance, with suspicious or mismatched scores).
That being said, Kellett is probably a good place to send your children to strong independent/public schools in the UK for GCSEs and/or sixth form, but not a smart choice to move to higher education from.
I hope this helps, but these are things in the long-term parents really ought to consider - if anything at all really.