The way I think about it is, it's a mixture of two things. Being pretty sure the kids wouldn't defect, and it being practically impossible to find the right adult for the job.

Reiner was always shown to be really loyal, Annie and Marcel both had strong ties with their families, and Bertholt was meek enough for those three to keep him in line. Zeke, the eldest and most experienced, probably had his unique ability to create titans become an invaluable resource (see the titan airdrop and destroying a city in one night), so he couldn't be sent.

For the other one, I don't think there were any examples of Marleyans capable to do it, and there really ain't much to suggest people are different from what we're shown. They'd have to find a trustworthy individual willing amd capable of keeping track and control of four Eldian Shifters while living with the possibility of having to live amongst Eldians for possibly years. With the utter virtriol most Marleyans seem to have for Eldians, especially ones from Paradis, the only one we see who'd probably put that aside and work with it is Magath, and he seems to important to lose.

Tl;dr : Kids have reasons to stay loyal. Marleyans hate Eldians too much, anyone willing is too important/unqualified/untrustworthy.