I believe that the frog stretch you are referring to, is the one laying on the belly and pulling the feet into a butterfly position trying to get the pelvis and feet to the floor, correct?
I have taught for 20 years and have also worked in physical therapy with concentration on dance medicine therapy. I teach stretch classes on a regular basis, so if you have more questions, just ask
From a basic anatomy point of view, the controversy lies in the fact that there is nothing about this stretch that is beneficial to a dancer-Especially a young one. It goes against everything that we teach them. Their backs have a slight sway in them, their feet are sickled, their pelvis is tilted the wrong way and their hips are not rotated outward. If you could lie a dancer down in this stretch and then lift them with a crane without their formation changing, you would be mortified in what you saw
The better alternative to do with an older student, of let's say 10-12, that have a better understanding of their bodies, their alignment and their core strength, would be to have them lie on their backs with their legs in the butterfly position. Then, their arms rest flat against the floor-palms down to help maintain balance. As they lift their hips off of the floor to flatten their spine/core-Similar to a bridge, their heels press up to the ceiling. So, the only parts of the body that remain on the floor are the toes, arms, shoulders and head. This allows the spine, pelvis and core to remain supported and strong. It also allows the hip flexor to get the necessary stretch/open position and the feet to not sickle.
I do not work on splits before the age of 9, unless they are on the competitive team. Only then, will I work on the basic formation of the stretch by age 7.5. I prefer runner's stretches over splits for the younger ages. Same goes for straddles/center stretches. We will sit on the floor and do these stretches lightly, but before the ages I mentioned, I just lightly correct their placement and move on to the next person. By the time I have gotten to the next person in line, they has lost their turnout already-Which in itself, is a sign that the age group or individual is just not able to grasp the concept yet. So I would observe your class and base your judgement off of the majority. If they can have proper seated alignment in a straddle, then you can slowly add in the stretch
As for a grand plie, I again do not do them with ages younger than 9 unless they are on my competitive team and have shown proper alignment in a demi plie for a full year in positions 1-3. During the next year, I will add an open 4th position for demi plie (I never do grand plies in 4th position-Not even in my advanced classes),and by January of that year, I may introduce the 5th position if they have mastered their turnout in 3rd and can fully engage the leg muscles while maintaining turnout? So no...I would not do a grand plie with an 8 year old recreational dancer.
I would be sure to include demi-pointe for tendu exercises and slowly working to the full tendu...
Good luck!! I love that you are organizing a syllabus and asking for feedback on what works and what doesn't

A syllabus is a GREAT tool!!