I don't think your son and daughter are going to face any problem while appearing for 10th board. Seeking admission, in the beginning, is the most difficult part of the entire process. Seeking admission in L.K.G. or 1st standard is a troublesome task. They are already in and there is no need to worry about anything.
Yes, RTE has fixed the minimum age for seeking admission in 1st standard as 5 years and 8 months for the academic year 2018-2019. Since your son is already in 1st standard, you don't have to worry about the new rules. Rules keep changing and the new rules are for those students who are enrolling this year in the schools. A new set of rules are not for those who have already reached higher levels.
Every year we see parents and management going through the same trouble. This year, however, an exact age is set which will help to enroll the students in the 2018-2019 academic years. Last year the minimum age for admission in 1st standard was 5 years and 4 months. The education department is trying to set the age limit to 6 years and thus the board is gradually increasing age limit every year.
Thus, do remember that whatever new set of rules are been introduced this year or will be introduced in the coming years will not apply to those students who have reached higher levels. The newly introduced rules only apply to new admissions.