Consider: Jordan and Sara tag along with Rachel and Cassie to the mall that day. (Or: where the Animorphs gets two kid tag alongs and Jordan hates to be called that)
I love the idea of Rachel’s big-sisterdom coming out 100x more than the little bits we see in canon when she’s interacting with Jake. However, I don’t think that her sisters being present at the construction site would end with Jordan and Sara becoming Animorphs. As of the first book, Jordan is ~10 years old and Sara is ~5.
I know that Elfangor proves willing to give the morphing tech to kids as young as 13, but I also suspect that he would draw the line at recruiting children that young. Assuming he didn’t, and assuming neither Jake nor Rachel stopped Jordan and Sara from using experimental alien technology, then they’d be asking a 5-year-old to keep a major secret from her own mother and teachers and friends. Not only that, but Sara would have to actively lie to her parents and friends, and do so in a way convincing enough to avoid any controllers becoming too suspicious. No offense to the 5-year-olds of the world, but the majority of them simply don’t have the ability to do that kind of counterfactual thinking.
Assuming that Sara and Jordan were allowed to morph, and assuming that they did manage to keep the whole thing a secret, then Rachel would never in a million years allow either of them into combat. Rachel’s a naturally protective person, and threats to her family tend to be her berserk button; she goes from viewing David as an enemy to viewing David as vermin that must be exterminated at all costs around the time David implies a threat to Jordan (#22), she just about loses her mind when she thinks that their collapsing house might have crushed her sisters (#12), and she refuses to entertain the possibility of letting them fight in #50. I don’t think that she would ever allow either of them to go into combat, no matter how desperate the situation.
Assuming that they do morph, they do keep the secret, and they do manage to nag Rachel into letting them fight… Hate to say it, but Jake is also too damn sensible to entertain the possibility. No way would he take on the responsibility of leading them into combat.
So while the idea is highly intriguing, I just don’t see a way to make it happen.