I would rather 'skilled youth spotters' rather than loyal players.
Give up something like;
"One of your coaches has been out into the local youth tournament and found a couple of promising players, they are currently 15 years old but would be able to join your team at the start of next season"
Give them 3 or 4 stars on the high skills and 2 or 3 stars on the low skills. Importantly give them 3 or 4 stars on the training speed.
Also make it so that they cannot be sold until 23 - they will be good but you will be stuck with them if you ever want rid you have to think long term.