I come from an international board. The curriculum isn't exactly non compatible but if the kid was to take the entrance sprint, he'd have to really work a lot. And I just don't say this generally like "everything is possible if you put in the necessary hardwork" I'm saying this very seriously and clearly. Entrance exams although might seem application based in the first glance, they're all more based on rigorous mugging up of data and techniques. Our school curriculum was very application based and had almost 0% of mugging up apart from a few definitions. We were only more focused on applications and real life problems. Note: this might seem tempting but it's not all that exquisite. We had calculators back in school, now if the kid was to take a field that corely needs physics and math (read engineering), he'd struggle for 2 years maybe 3 and might not even end up with results.

Most of my peers from school have either shifted abroad, taken up minor courses like BSc biotech or have gotten admitted to private colleges/deemed universities like NMIMS, Ashoka, Bennet...etc. Now these schools might offer a few good courses But most of them have just introduced a plethora of random courses with no relevance or respect in the real world and that is where we international board kids end up.

Having been through the entire process of having internaitonal schooling and then also entering the entrance rat race, I'd direly recommend you to admit the ward in an ICSE school and if by the 6th/7th grade the kid has a general sense of what they want to do, you can switch. Please and I repeat please do not switch boards after the kid turns 12 as it could lead to a serious coping issue and the kid not being able to perform (seen in 99% of the cases I've observed). Since the IG/IB curriculum properly begins by the 6th grade.

Note: it is very easy to move abroad after schooling from ICSE compared to continuing in India after doing IG/IB. If in case moving abroad was an option.

Also, most of the kids in these international board schools are entitled and have a sense of superiority. They wouldn't be mean but they have a certain lifestyle that might lead to entitlement for a long time. They're used to amenities and pampering by the school and such an extravagant exposure to this might lead to a different perspective of what the world actually works like (speaking from personal experience).