Ken Prep and Thomas’s are very different schools, on many dimensions, starting from academics. And it’s quite short sighted to make a choice between them based on drop off logistics. In my opinion.
I’ll qualify this with the assumption that you are in the U.K. education system for the long term and your daughter will be applying to the secondary schools in London at 11+.
If it’s not the case - then yes, convenience matters more and getting kids to the same school can be the main criteria.
But - if you are based here, and are going to do 11+ with your daughter - I’d not pass on KP. KP is simply better for an academic girl to get to her fuller potential. And have better outcomes at 11+ as a result.
And don’t forget that for your boy you may be chose ing between 7/8/11/13+... So you don’t even know how long he will be in his first prep.
Generally, it is quite hard to teach both 4yo boys and girls together effectively. They are just too different at that age.
And then of course there is the simple fact of 11+ being structured so differently for boys and girls - this making it’s further harder to teach them together at Y5/6 to have good results.
Many people in this area do two drop offs for boys/girls schools. Or, rather it starts with nursery/school, then moves on.
Most holidays do overlap, and if they don’t - it’s only sometimes and for some (parts of) half-terms, that is manageable. Just a fact of life.
And mind you - your kids would probably be in different secondaries at some point - and holidays may differ anyway.
Logistics do matter to en extent, obviously. But most can be resolved - schools have earlier drop offs; people cooperate with other parents; childminders - etc.